feat: initial Silent Send browser extension

Chrome Manifest V3 extension that intercepts personal data and
substitutes it with user-defined replacements before sending to
Claude.ai. Hooks fetch() in the page's main world to catch API
requests, with bidirectional substitution (real→fake on send,
fake→real on display via reveal mode).

Includes popup UI with mapping management, live test/diff view,
activity log with badge count, options page with import/export,
and Shadow DOM traversal for Claude.ai compatibility.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/background/service-worker.js

@@ -0,0 +1,103 @@
+/**
+ * Silent Send - Background Service Worker
+ *
+ * Manages badge count, coordinates between popup and content scripts.
+ */
+
+import Storage from '../lib/storage.js';
+
+// Track substitution counts per tab
+const tabCounts = new Map();
+
+// --- Badge Management ---
+
+function updateBadge(tabId) {
+  const count = tabCounts.get(tabId) || 0;
+  const text = count > 0 ? String(count) : '';
+
+  chrome.action.setBadgeText({ text, tabId });
+  chrome.action.setBadgeBackgroundColor({ color: count > 0 ? '#10b981' : '#6b7280', tabId });
+}
+
+// Reset count when tab navigates
+chrome.tabs.onUpdated.addListener((tabId, changeInfo) => {
+  if (changeInfo.status === 'loading') {
+    tabCounts.set(tabId, 0);
+    updateBadge(tabId);
+  }
+});
+
+// Cleanup when tab closes
+chrome.tabs.onRemoved.addListener((tabId) => {
+  tabCounts.delete(tabId);
+});
+
+// --- Message Handling ---
+
+chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
+  const handler = messageHandlers[message.type];
+  if (handler) {
+    handler(message, sender, sendResponse);
+    return true; // async response
+  }
+});
+
+const messageHandlers = {
+  async 'substitution:performed'(message, sender) {
+    const tabId = sender.tab?.id;
+    if (tabId == null) return;
+
+    const current = tabCounts.get(tabId) || 0;
+    tabCounts.set(tabId, current + message.count);
+    updateBadge(tabId);
+
+    // Log each replacement
+    for (const replacement of message.replacements) {
+      await Storage.addLogEntry({
+        type: 'substitution',
+        direction: 'outbound',
+        original: replacement.original,
+        replaced: replacement.replaced,
+        category: replacement.category,
+        url: sender.tab?.url || '',
+      });
+    }
+  },
+
+  async 'get:mappings'(_message, _sender, sendResponse) {
+    const mappings = await Storage.getMappings();
+    sendResponse({ mappings });
+  },
+
+  async 'get:settings'(_message, _sender, sendResponse) {
+    const settings = await Storage.getSettings();
+    sendResponse({ settings });
+  },
+
+  async 'get:log'(_message, _sender, sendResponse) {
+    const log = await Storage.getLog();
+    sendResponse({ log });
+  },
+
+  async 'get:tab-count'(message, sender, sendResponse) {
+    const tabId = message.tabId || sender.tab?.id;
+    sendResponse({ count: tabCounts.get(tabId) || 0 });
+  },
+
+  async 'update:settings'(message) {
+    await Storage.saveSettings(message.settings);
+    // Broadcast to content scripts
+    const tabs = await chrome.tabs.query({ url: 'https://claude.ai/*' });
+    for (const tab of tabs) {
+      chrome.tabs.sendMessage(tab.id, {
+        type: 'settings:updated',
+        settings: message.settings,
+      }).catch(() => {});
+    }
+  },
+};
+
+// --- Set initial badge state ---
+chrome.runtime.onInstalled.addListener(() => {
+  chrome.action.setBadgeBackgroundColor({ color: '#6b7280' });
+});