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/content/injector.js

@@ -0,0 +1,60 @@
+/**
+ * Silent Send - Content Script Injector
+ *
+ * This runs in Chrome's ISOLATED content script world, where it has
+ * access to chrome.storage. It then injects the fetch-hooking code
+ * into the MAIN page world so it can intercept the actual fetch() calls.
+ *
+ * Communication: page script <-> content script via window.postMessage
+ */
+
+'use strict';
+
+// Load mappings and settings, then inject into page
+async function init() {
+  const result = await chrome.storage.local.get(['ss_mappings', 'ss_settings']);
+  const mappings = result.ss_mappings || [];
+  const settings = result.ss_settings || { enabled: true };
+
+  // Inject the main interception script into the page's world
+  const script = document.createElement('script');
+  script.setAttribute('data-ss-config', JSON.stringify({ mappings, settings }));
+  script.src = chrome.runtime.getURL('src/content/content.js');
+  (document.head || document.documentElement).appendChild(script);
+  script.onload = () => script.remove();
+
+  // Listen for substitution events from the page script
+  window.addEventListener('message', (event) => {
+    if (event.source !== window) return;
+    if (event.data?.type === 'ss:substitution-performed') {
+      chrome.runtime.sendMessage({
+        type: 'substitution:performed',
+        count: event.data.count,
+        replacements: event.data.replacements,
+      }).catch(() => {});
+    }
+  });
+
+  // Forward storage changes to the page script
+  chrome.storage.onChanged.addListener((changes) => {
+    if (changes.ss_mappings || changes.ss_settings) {
+      window.postMessage({
+        type: 'ss:config-updated',
+        mappings: changes.ss_mappings?.newValue,
+        settings: changes.ss_settings?.newValue,
+      }, '*');
+    }
+  });
+
+  // Listen for settings updates from popup via runtime messages
+  chrome.runtime.onMessage.addListener((message) => {
+    if (message.type === 'settings:updated') {
+      window.postMessage({
+        type: 'ss:config-updated',
+        settings: message.settings,
+      }, '*');
+    }
+  });
+}
+
+init();