diff --git a/manifest.firefox.json b/manifest.firefox.json
index 31e9417..b20038a 100644
--- a/manifest.firefox.json
+++ b/manifest.firefox.json
@@ -12,7 +12,8 @@
   "permissions": [
     "storage",
     "activeTab",
-    "scripting"
+    "scripting",
+    "notifications"
   ],
   "host_permissions": [
     "https://claude.ai/*",
diff --git a/manifest.json b/manifest.json
index f16e63c..d73335f 100644
--- a/manifest.json
+++ b/manifest.json
@@ -6,7 +6,8 @@
   "permissions": [
     "storage",
     "activeTab",
-    "scripting"
+    "scripting",
+    "notifications"
   ],
   "host_permissions": [
     "https://claude.ai/*",
diff --git a/src/background/service-worker.js b/src/background/service-worker.js
index 7f04898..a93cb58 100644
--- a/src/background/service-worker.js
+++ b/src/background/service-worker.js
@@ -146,6 +146,13 @@ const messageHandlers = {
     sendResponse({ count: tabCounts.get(tabId) || 0 });
   },
 
+  async 'sync:notification-seen'() {
+    // Options page opened — clear the sync badge and pending notification flag
+    await api.storage.local.remove('ss_sync_notification');
+    api.action.setBadgeText({ text: '' });
+    api.action.setBadgeBackgroundColor({ color: '#6b7280' });
+  },
+
   async 'update:settings'(message) {
     await Storage.saveSettings(message.settings);
 
@@ -309,6 +316,33 @@ api.storage.onChanged.addListener(async (changes, areaName) => {
     }
   }
 
+  // When a sync operation applied new data, show badge + notification
+  if (areaName === 'local' && changes.ss_sync_notification?.newValue) {
+    const notif = changes.ss_sync_notification.newValue;
+    const sourceLabel = {
+      'file': 'sync folder',
+      'browser-sync': 'browser account sync',
+      'code': 'sync code import',
+    }[notif.source] || 'sync';
+
+    // Purple badge — persists until Options is opened
+    api.action.setBadgeText({ text: 'SYN' });
+    api.action.setBadgeBackgroundColor({ color: '#7c3aed' });
+
+    // Desktop notification
+    try {
+      api.notifications.create('ss-sync-applied', {
+        type: 'basic',
+        iconUrl: 'icons/icon48.svg',
+        title: 'Silent Send — Settings Synced',
+        message: `Settings updated via ${sourceLabel}. Open Options to review.`,
+        priority: 1,
+      });
+    } catch (e) {
+      // Notifications permission not granted — badge is still visible
+    }
+  }
+
   // When sync storage changes (another device pushed new data), pull it into local
   if (areaName === 'sync' && (changes.ss_sync_meta || Object.keys(changes).some(k => k.startsWith('ss_sync_chunk_')))) {
     const settings = await Storage.getSettings();
@@ -318,6 +352,14 @@ api.storage.onChanged.addListener(async (changes, areaName) => {
   }
 });
 
+// Clicking a sync notification opens the Options page
+api.notifications.onClicked.addListener((notificationId) => {
+  if (notificationId === 'ss-sync-applied') {
+    api.runtime.openOptionsPage();
+    api.notifications.clear(notificationId);
+  }
+});
+
 // --- Set initial state ---
 api.runtime.onInstalled.addListener(async () => {
   api.action.setBadgeBackgroundColor({ color: '#6b7280' });
@@ -329,6 +371,14 @@ api.runtime.onInstalled.addListener(async () => {
 (async () => {
   const settings = await Storage.getSettings();
   await updateIcon(settings);
+
+  // Restore the SYN badge if the user hasn't opened Options since the last sync
+  const stored = await api.storage.local.get('ss_sync_notification');
+  if (stored.ss_sync_notification) {
+    api.action.setBadgeText({ text: 'SYN' });
+    api.action.setBadgeBackgroundColor({ color: '#7c3aed' });
+  }
+
   if (settings.browserSync) {
     await SilentSendSync.pullFromSyncStorage();
   }
diff --git a/src/content/content.js b/src/content/content.js
index ec9b49f..0f25436 100644
--- a/src/content/content.js
+++ b/src/content/content.js
@@ -1134,9 +1134,17 @@
         });
         await setStorageData('ss_mappings', currentMappings);
 
-        // Update local mappings
+        // Update local mappings so the fetch interceptor uses them immediately
         mappings = currentMappings;
 
+        // Replace the PPI value in the current input right now
+        if (inputEl) {
+          replaceInInput(inputEl, real, fake);
+          // Re-scan — will dismiss warning if no more PPI remains
+          if (inputScanTimer) clearTimeout(inputScanTimer);
+          inputScanTimer = setTimeout(() => scanInputForPPI(inputEl), 150);
+        }
+
         // Visual feedback
         btn.textContent = '\u2714';
         btn.style.color = '#4ade80';
@@ -1145,6 +1153,27 @@
     });
   }
 
+  // Replace all occurrences of `real` with `fake` in an input or contenteditable element
+  function replaceInInput(el, real, fake) {
+    if (!el) return;
+    const re = new RegExp(real.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'), 'gi');
+    if (el.value !== undefined) {
+      // <textarea> or <input>
+      el.value = el.value.replace(re, fake);
+    } else if (el.isContentEditable) {
+      // contenteditable div — walk text nodes to avoid breaking inner HTML
+      const walker = document.createTreeWalker(el, NodeFilter.SHOW_TEXT);
+      let node;
+      while ((node = walker.nextNode())) {
+        re.lastIndex = 0;
+        if (re.test(node.textContent)) {
+          re.lastIndex = 0;
+          node.textContent = node.textContent.replace(re, fake);
+        }
+      }
+    }
+  }
+
   // Storage helpers for page world (uses postMessage to injector)
   function getStorageData(key) {
     return new Promise(resolve => {
diff --git a/src/lib/sync.js b/src/lib/sync.js
index ccdea73..1c6368c 100644
--- a/src/lib/sync.js
+++ b/src/lib/sync.js
@@ -1,14 +1,16 @@
 /**
  * Silent Send - Cross-Browser Settings Sync
  *
- * Two sync mechanisms:
+ * Sync mechanisms:
  *
- * 1. Sync Code — base64-encoded JSON snapshot for manual copy-paste between
- *    browsers (works across any browser/device combination).
- *
- * 2. browser.storage.sync — automatic sync within the same browser family
- *    (Firefox ↔ Firefox via Firefox Sync, Chrome ↔ Chrome via Google account).
- *    Data is chunked to stay within per-item size limits.
+ * 1. Sync Code — base64-encoded JSON snapshot for manual copy-paste.
+ * 2. browser.storage.sync — automatic within the same browser family.
+ * 3. Folder sync (File System Access API) — any locally-mounted folder
+ *    including Dropbox, OneDrive, Google Drive, iCloud, Nextcloud, etc.
+ * 4. GitHub Gist — serverless cloud sync using a personal access token;
+ *    works across any browser/device without a local desktop client.
+ * 5. Custom HTTP endpoint — any URL supporting GET + PUT (WebDAV,
+ *    self-hosted server, cloud function, etc.).
  *
  * Conflict resolution: newest `lastModified` timestamp wins.
  */
@@ -65,7 +67,7 @@ const SilentSendSync = {
         }
       }
 
-      await this._applyData(data);
+      await this._applyData(data, 'code');
       return { success: true, importTime: new Date(data.lastModified).toLocaleString() };
     } catch (e) {
       return { success: false, reason: e.message };
@@ -131,7 +133,7 @@ const SilentSendSync = {
       const json = chunkKeys.map(k => chunkResult[k] || '').join('');
       const data = JSON.parse(json);
 
-      await this._applyData(data);
+      await this._applyData(data, 'browser-sync');
       return { imported: true, time: new Date(data.lastModified).toLocaleString() };
     } catch (e) {
       console.warn('[Silent Send] pullFromSyncStorage failed:', e);
@@ -154,8 +156,12 @@ const SilentSendSync = {
     };
   },
 
-  async _applyData(data) {
-    const toSet = { ss_lastModified: data.lastModified };
+  async _applyData(data, source = 'unknown') {
+    const toSet = {
+      ss_lastModified: data.lastModified,
+      // Signal the service worker to show a badge/notification
+      ss_sync_notification: { source, time: Date.now() },
+    };
     if (data.identity !== undefined) toSet.ss_identity = data.identity;
     if (data.mappings !== undefined) toSet.ss_mappings = data.mappings;
     if (data.settings !== undefined) toSet.ss_settings = data.settings;
@@ -171,6 +177,205 @@ const SilentSendSync = {
       return [];
     }
   },
+
+  // ----------------------------------------------------------------
+  // GitHub Gist sync
+  // Requires a personal access token with the `gist` scope.
+  // On first push a new secret Gist is created; the Gist ID is stored
+  // in local storage so all subsequent reads/writes use the same Gist.
+  // ----------------------------------------------------------------
+
+  /**
+   * Push current data to a GitHub Gist (creates one if no gist ID stored).
+   * token: GitHub PAT with `gist` scope.
+   * Returns { success, gistId } or { success: false, reason }.
+   */
+  async pushToGist(token) {
+    if (!token) return { success: false, reason: 'No GitHub token provided.' };
+    try {
+      const data = await this._getAllData();
+      const content = JSON.stringify(data, null, 2);
+      const stored = await api.storage.local.get('ss_gist_id');
+      const gistId = stored.ss_gist_id;
+
+      let resp;
+      if (gistId) {
+        // Update existing Gist
+        resp = await fetch(`https://api.github.com/gists/${gistId}`, {
+          method: 'PATCH',
+          headers: { Authorization: `token ${token}`, 'Content-Type': 'application/json' },
+          body: JSON.stringify({ files: { 'silent-send-sync.json': { content } } }),
+        });
+      } else {
+        // Create new secret Gist
+        resp = await fetch('https://api.github.com/gists', {
+          method: 'POST',
+          headers: { Authorization: `token ${token}`, 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            description: 'Silent Send settings sync',
+            public: false,
+            files: { 'silent-send-sync.json': { content } },
+          }),
+        });
+      }
+
+      if (!resp.ok) {
+        const err = await resp.json().catch(() => ({}));
+        return { success: false, reason: err.message || `HTTP ${resp.status}` };
+      }
+
+      const json = await resp.json();
+      await api.storage.local.set({ ss_gist_id: json.id });
+      return { success: true, gistId: json.id };
+    } catch (e) {
+      return { success: false, reason: e.message };
+    }
+  },
+
+  /**
+   * Pull data from a GitHub Gist and apply if newer.
+   * token: GitHub PAT with `gist` scope.
+   * Returns { success, imported?, reason? }.
+   */
+  async pullFromGist(token) {
+    if (!token) return { success: false, reason: 'No GitHub token provided.' };
+    try {
+      const stored = await api.storage.local.get('ss_gist_id');
+      const gistId = stored.ss_gist_id;
+      if (!gistId) return { success: false, reason: 'No Gist ID stored. Push first.' };
+
+      const resp = await fetch(`https://api.github.com/gists/${gistId}`, {
+        headers: { Authorization: `token ${token}` },
+      });
+      if (!resp.ok) {
+        const err = await resp.json().catch(() => ({}));
+        return { success: false, reason: err.message || `HTTP ${resp.status}` };
+      }
+
+      const gist = await resp.json();
+      const file = gist.files?.['silent-send-sync.json'];
+      if (!file) return { success: false, reason: 'Sync file not found in Gist.' };
+
+      // Fetch raw content (may be truncated in the API response)
+      const rawResp = await fetch(file.raw_url);
+      const data = JSON.parse(await rawResp.text());
+
+      const local = await this._getAllData();
+      if (data.lastModified <= (local.lastModified || 0)) {
+        return { success: true, imported: false };
+      }
+
+      await this._applyData(data, 'gist');
+      return { success: true, imported: true, time: new Date(data.lastModified).toLocaleString() };
+    } catch (e) {
+      return { success: false, reason: e.message };
+    }
+  },
+
+  // ----------------------------------------------------------------
+  // Custom HTTP endpoint sync
+  // GET fetches the JSON, PUT/PATCH writes it.
+  // Works with WebDAV (Nextcloud, ownCloud), any REST endpoint, or a
+  // simple static file server that supports PUT.
+  // ----------------------------------------------------------------
+
+  /**
+   * Push to a custom URL via HTTP PUT.
+   * opts: { url, method = 'PUT', headers = {} }
+   */
+  async pushToUrl({ url, method = 'PUT', headers = {} } = {}) {
+    if (!url) return { success: false, reason: 'No URL provided.' };
+    try {
+      const data = await this._getAllData();
+      const resp = await fetch(url, {
+        method,
+        headers: { 'Content-Type': 'application/json', ...headers },
+        body: JSON.stringify(data, null, 2),
+      });
+      if (!resp.ok) return { success: false, reason: `HTTP ${resp.status}` };
+      return { success: true };
+    } catch (e) {
+      return { success: false, reason: e.message };
+    }
+  },
+
+  /**
+   * Pull from a custom URL via HTTP GET and apply if newer.
+   * opts: { url, headers = {} }
+   */
+  async pullFromUrl({ url, headers = {} } = {}) {
+    if (!url) return { success: false, reason: 'No URL provided.' };
+    try {
+      const resp = await fetch(url, { headers });
+      if (!resp.ok) return { success: false, reason: `HTTP ${resp.status}` };
+      const data = await resp.json();
+
+      const local = await this._getAllData();
+      if (data.lastModified <= (local.lastModified || 0)) {
+        return { success: true, imported: false };
+      }
+
+      await this._applyData(data, 'url');
+      return { success: true, imported: true, time: new Date(data.lastModified).toLocaleString() };
+    } catch (e) {
+      return { success: false, reason: e.message };
+    }
+  },
+
+  // ----------------------------------------------------------------
+  // File System Access API helpers — folder-based sync
+  // The directory handle is stored in IndexedDB so the user only
+  // needs to grant access once per browser session.
+  // ----------------------------------------------------------------
+
+  _dbPromise: null,
+
+  _openDB() {
+    if (this._dbPromise) return this._dbPromise;
+    this._dbPromise = new Promise((resolve, reject) => {
+      const req = indexedDB.open('ss_sync_handles', 1);
+      req.onupgradeneeded = () => req.result.createObjectStore('handles');
+      req.onsuccess = () => resolve(req.result);
+      req.onerror = () => reject(req.error);
+    });
+    return this._dbPromise;
+  },
+
+  async saveSyncDirHandle(handle) {
+    const db = await this._openDB();
+    return new Promise((resolve, reject) => {
+      const tx = db.transaction('handles', 'readwrite');
+      tx.objectStore('handles').put(handle, 'syncDir');
+      tx.oncomplete = resolve;
+      tx.onerror = () => reject(tx.error);
+    });
+  },
+
+  async loadSyncDirHandle() {
+    try {
+      const db = await this._openDB();
+      return new Promise((resolve) => {
+        const tx = db.transaction('handles', 'readonly');
+        const req = tx.objectStore('handles').get('syncDir');
+        req.onsuccess = () => resolve(req.result || null);
+        req.onerror = () => resolve(null);
+      });
+    } catch {
+      return null;
+    }
+  },
+
+  async clearSyncDirHandle() {
+    try {
+      const db = await this._openDB();
+      return new Promise((resolve) => {
+        const tx = db.transaction('handles', 'readwrite');
+        tx.objectStore('handles').delete('syncDir');
+        tx.oncomplete = resolve;
+        tx.onerror = resolve;
+      });
+    } catch { /* ignore */ }
+  },
 };
 
 export default SilentSendSync;
diff --git a/src/options/options.html b/src/options/options.html
index 4589afa..03a2eb4 100644
--- a/src/options/options.html
+++ b/src/options/options.html
@@ -140,6 +140,64 @@
 
         <div id="syncStatus" style="margin-top:8px;font-size:12px;min-height:16px"></div>
       </div>
+
+      <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
+        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Auto-Sync Folder — fully automatic, no copy-paste</h3>
+        <p class="section-desc" style="margin-bottom:8px">
+          Pick the same folder in each browser once. Changes are written to
+          <code>silent-send-sync.json</code> automatically whenever settings change,
+          and read back whenever this page is opened or regains focus.
+          Works with <strong>any cloud storage that has a desktop sync client</strong> —
+          Dropbox, OneDrive, Google Drive, iCloud Drive, Box, pCloud, Nextcloud,
+          Synology Drive, or any network share. Just pick the cloud-synced folder.
+        </p>
+        <div class="bulk-actions" style="align-items:center">
+          <button class="btn btn-primary" id="btnPickSyncFolder">Choose Sync Folder</button>
+          <span id="syncFolderName" style="font-size:12px;color:#6b7280;font-family:monospace"></span>
+          <button class="btn btn-danger btn-sm" id="btnClearSyncFolder" style="display:none">Clear</button>
+        </div>
+        <div id="fileSyncStatus" style="margin-top:6px;font-size:12px;min-height:16px"></div>
+      </div>
+
+      <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
+        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">GitHub Gist Sync — no desktop client needed</h3>
+        <p class="section-desc" style="margin-bottom:8px">
+          Store settings in a private GitHub Gist. Works across any browser or device
+          with just a GitHub account. Create a
+          <a href="https://github.com/settings/tokens/new?scopes=gist&description=SilentSend" target="_blank" rel="noopener">
+            Personal Access Token
+          </a>
+          with the <code>gist</code> scope, paste it below, and click Push.
+          The Gist ID is remembered — future pushes update the same Gist.
+        </p>
+        <div style="display:flex;gap:8px;margin-bottom:6px;flex-wrap:wrap">
+          <input type="password" id="gistToken" placeholder="GitHub PAT (ghp_…)" autocomplete="off"
+            style="flex:1;min-width:180px;font-family:monospace;font-size:12px;padding:5px 8px;border:1px solid #d1d5db;border-radius:6px">
+          <button class="btn btn-primary" id="btnGistPush">Push</button>
+          <button class="btn" id="btnGistPull">Pull</button>
+        </div>
+        <div id="gistSyncStatus" style="font-size:12px;min-height:16px"></div>
+      </div>
+
+      <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
+        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Custom URL Sync</h3>
+        <p class="section-desc" style="margin-bottom:8px">
+          Any URL that supports HTTP GET (read) and PUT (write). Works with
+          Nextcloud/ownCloud WebDAV, a self-hosted server, or a cloud function.
+          Add custom headers (e.g. Authorization) as JSON if needed.
+        </p>
+        <div style="display:flex;gap:8px;margin-bottom:6px;flex-wrap:wrap">
+          <input type="url" id="customSyncUrl" placeholder="https://…/silent-send-sync.json"
+            style="flex:2;min-width:200px;font-size:12px;padding:5px 8px;border:1px solid #d1d5db;border-radius:6px">
+          <input type="text" id="customSyncHeaders" placeholder='{"Authorization":"Bearer …"}' autocomplete="off"
+            style="flex:1;min-width:160px;font-family:monospace;font-size:11px;padding:5px 8px;border:1px solid #d1d5db;border-radius:6px">
+        </div>
+        <div class="bulk-actions">
+          <button class="btn btn-primary" id="btnUrlPush">Push</button>
+          <button class="btn" id="btnUrlPull">Pull</button>
+        </div>
+        <div id="urlSyncStatus" style="margin-top:6px;font-size:12px;min-height:16px"></div>
+      </div>
     </section>
 
     <section class="section">
diff --git a/src/options/options.js b/src/options/options.js
index 2bad103..626fc3c 100644
--- a/src/options/options.js
+++ b/src/options/options.js
@@ -95,6 +95,107 @@ document.addEventListener('DOMContentLoaded', async () => {
     setSyncStatus('', 'neutral');
   });
 
+  // --- File-based auto-sync ---
+  // Tell the service worker the user has seen any pending sync notification
+  api.runtime.sendMessage({ type: 'sync:notification-seen' }).catch(() => {});
+  await api.storage.local.remove('ss_sync_notification');
+
+  await initFileSync();
+
+  $('#btnPickSyncFolder').addEventListener('click', pickSyncFolder);
+  $('#btnClearSyncFolder').addEventListener('click', async () => {
+    await SilentSendSync.clearSyncDirHandle();
+    syncDirHandle = null;
+    updateFileSyncUI();
+    setFileSyncStatus('Sync folder cleared.', 'neutral');
+  });
+
+  // Auto-write when local storage changes (catches popup edits, page-world adds, etc.)
+  api.storage.onChanged.addListener(async (changes, area) => {
+    if (area === 'local' && (changes.ss_settings || changes.ss_mappings || changes.ss_identity)) {
+      await writeToSyncFile();
+    }
+  });
+
+  // Re-check sync file whenever the options page regains focus
+  window.addEventListener('focus', async () => {
+    await checkFileSyncUpdate();
+  });
+
+  // --- GitHub Gist sync ---
+  // Restore saved token (session only — never persisted to storage)
+  {
+    const stored = await api.storage.local.get('ss_gist_id');
+    if (stored.ss_gist_id) {
+      setGistSyncStatus(`Gist ID: ${stored.ss_gist_id.slice(0, 12)}…`, 'ok');
+    }
+  }
+
+  $('#btnGistPush').addEventListener('click', async () => {
+    const token = $('#gistToken').value.trim();
+    if (!token) { setGistSyncStatus('Enter your GitHub PAT first.', 'warn'); return; }
+    setGistSyncStatus('Pushing…', 'neutral');
+    const r = await SilentSendSync.pushToGist(token);
+    if (r.success) {
+      setGistSyncStatus(`Pushed. Gist ID: ${r.gistId.slice(0, 12)}…`, 'ok');
+    } else {
+      setGistSyncStatus('Push failed: ' + r.reason, 'error');
+    }
+  });
+
+  $('#btnGistPull').addEventListener('click', async () => {
+    const token = $('#gistToken').value.trim();
+    if (!token) { setGistSyncStatus('Enter your GitHub PAT first.', 'warn'); return; }
+    setGistSyncStatus('Pulling…', 'neutral');
+    const r = await SilentSendSync.pullFromGist(token);
+    if (!r.success) {
+      setGistSyncStatus('Pull failed: ' + r.reason, 'error');
+    } else if (r.imported) {
+      setGistSyncStatus(`Pulled (${r.time}). Refreshing…`, 'ok');
+      mappings = await Storage.getMappings();
+      settings = await Storage.getSettings();
+      renderMappings();
+      renderDomains();
+      renderLog();
+    } else {
+      setGistSyncStatus('Already up to date.', 'ok');
+    }
+  });
+
+  // --- Custom URL sync ---
+  $('#btnUrlPush').addEventListener('click', async () => {
+    const url = $('#customSyncUrl').value.trim();
+    if (!url) { setUrlSyncStatus('Enter a URL first.', 'warn'); return; }
+    const headers = parseHeadersField($('#customSyncHeaders').value);
+    setUrlSyncStatus('Pushing…', 'neutral');
+    const r = await SilentSendSync.pushToUrl({ url, headers });
+    if (r.success) {
+      setUrlSyncStatus('Pushed successfully.', 'ok');
+    } else {
+      setUrlSyncStatus('Push failed: ' + r.reason, 'error');
+    }
+  });
+
+  $('#btnUrlPull').addEventListener('click', async () => {
+    const url = $('#customSyncUrl').value.trim();
+    if (!url) { setUrlSyncStatus('Enter a URL first.', 'warn'); return; }
+    const headers = parseHeadersField($('#customSyncHeaders').value);
+    setUrlSyncStatus('Pulling…', 'neutral');
+    const r = await SilentSendSync.pullFromUrl({ url, headers });
+    if (!r.success) {
+      setUrlSyncStatus('Pull failed: ' + r.reason, 'error');
+    } else if (r.imported) {
+      setUrlSyncStatus(`Pulled (${r.time}). Refreshing…`, 'ok');
+      mappings = await Storage.getMappings();
+      settings = await Storage.getSettings();
+      renderMappings();
+      renderDomains();
+      renderLog();
+    } else {
+      setUrlSyncStatus('Already up to date.', 'ok');
+    }
+  });
+
   // Transfer data
   $('#btnExportAll').addEventListener('click', exportAllPlain);
   $('#btnExportEncrypted').addEventListener('click', exportAllEncrypted);
@@ -465,6 +566,142 @@ async function importAll(e) {
   e.target.value = '';
 }
 
+// ----------------------------------------------------------------
+// File-based auto-sync (File System Access API)
+// ----------------------------------------------------------------
+
+let syncDirHandle = null;
+const SYNC_FILE_NAME = 'silent-send-sync.json';
+
+async function initFileSync() {
+  syncDirHandle = await SilentSendSync.loadSyncDirHandle();
+  updateFileSyncUI();
+  if (syncDirHandle) {
+    await checkFileSyncUpdate();
+  }
+}
+
+async function pickSyncFolder() {
+  if (!window.showDirectoryPicker) {
+    setFileSyncStatus('Your browser does not support the File System Access API.', 'error');
+    return;
+  }
+  try {
+    const handle = await window.showDirectoryPicker({ mode: 'readwrite', id: 'ss-sync' });
+    syncDirHandle = handle;
+    await SilentSendSync.saveSyncDirHandle(handle);
+    updateFileSyncUI();
+    // Write current settings immediately so the file exists for the other browser
+    await writeToSyncFile();
+    setFileSyncStatus('Sync folder set. Settings will sync automatically.', 'ok');
+  } catch (e) {
+    if (e.name !== 'AbortError') {
+      setFileSyncStatus('Could not set sync folder: ' + e.message, 'error');
+    }
+  }
+}
+
+async function writeToSyncFile() {
+  if (!syncDirHandle) return;
+  try {
+    // Re-verify permission is still granted (required after browser restart)
+    const perm = await syncDirHandle.requestPermission({ mode: 'readwrite' });
+    if (perm !== 'granted') return;
+
+    const data = await SilentSendSync._getAllData();
+    const fileHandle = await syncDirHandle.getFileHandle(SYNC_FILE_NAME, { create: true });
+    const writable = await fileHandle.createWritable();
+    await writable.write(JSON.stringify(data, null, 2));
+    await writable.close();
+  } catch (e) {
+    // Permission denied or folder removed — don't spam errors
+    console.warn('[Silent Send] writeToSyncFile failed:', e.message);
+  }
+}
+
+async function checkFileSyncUpdate() {
+  if (!syncDirHandle) return;
+  try {
+    const perm = await syncDirHandle.queryPermission({ mode: 'readwrite' });
+    if (perm === 'prompt') {
+      // Need a user gesture to re-request — skip silently
+      return;
+    }
+    if (perm !== 'granted') return;
+
+    const fileHandle = await syncDirHandle.getFileHandle(SYNC_FILE_NAME);
+    const file = await fileHandle.getFile();
+    const data = JSON.parse(await file.text());
+
+    if (!data.version || !data.lastModified) return;
+
+    const local = await SilentSendSync._getAllData();
+    if (data.lastModified > (local.lastModified || 0)) {
+      await SilentSendSync._applyData(data, 'file');
+      mappings = await Storage.getMappings();
+      settings = await Storage.getSettings();
+      $('#browserSync').checked = settings.browserSync === true;
+      renderMappings();
+      renderDomains();
+      renderLog();
+      setFileSyncStatus(
+        'Auto-synced from folder (' + new Date(data.lastModified).toLocaleString() + ').',
+        'ok'
+      );
+    }
+  } catch (e) {
+    if (e.name !== 'NotFoundError') {
+      console.warn('[Silent Send] checkFileSyncUpdate failed:', e.message);
+    }
+  }
+}
+
+function updateFileSyncUI() {
+  const nameEl = $('#syncFolderName');
+  const clearBtn = $('#btnClearSyncFolder');
+  const pickBtn = $('#btnPickSyncFolder');
+  if (!nameEl) return;
+  if (syncDirHandle) {
+    nameEl.textContent = syncDirHandle.name + '/';
+    clearBtn.style.display = '';
+    pickBtn.textContent = 'Change Folder';
+  } else {
+    nameEl.textContent = '';
+    clearBtn.style.display = 'none';
+    pickBtn.textContent = 'Choose Sync Folder';
+  }
+}
+
+function setFileSyncStatus(msg, type) {
+  const el = $('#fileSyncStatus');
+  if (!el) return;
+  el.textContent = msg;
+  el.style.color = type === 'ok' ? '#10b981' : type === 'warn' ? '#f59e0b' : type === 'error' ? '#dc2626' : '#6b7280';
+}
+
+function setGistSyncStatus(msg, type) {
+  const el = $('#gistSyncStatus');
+  if (!el) return;
+  el.textContent = msg;
+  el.style.color = type === 'ok' ? '#10b981' : type === 'warn' ? '#f59e0b' : type === 'error' ? '#dc2626' : '#6b7280';
+}
+
+function setUrlSyncStatus(msg, type) {
+  const el = $('#urlSyncStatus');
+  if (!el) return;
+  el.textContent = msg;
+  el.style.color = type === 'ok' ? '#10b981' : type === 'warn' ? '#f59e0b' : type === 'error' ? '#dc2626' : '#6b7280';
+}
+
+function parseHeadersField(val) {
+  if (!val || !val.trim()) return {};
+  try {
+    return JSON.parse(val.trim());
+  } catch {
+    return {};
+  }
+}
+
 function setSyncStatus(msg, type) {
   const el = $('#syncStatus');
   if (!el) return;