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

manifest.json

@@ -0,0 +1,47 @@
+{
+  "manifest_version": 3,
+  "name": "Silent Send",
+  "version": "0.1.0",
+  "description": "Intercepts personal info and substitutes it with user-defined replacements before sending to AI services.",
+  "permissions": [
+    "storage",
+    "activeTab",
+    "scripting"
+  ],
+  "host_permissions": [
+    "https://claude.ai/*"
+  ],
+  "background": {
+    "service_worker": "src/background/service-worker.js",
+    "type": "module"
+  },
+  "content_scripts": [
+    {
+      "matches": ["https://claude.ai/*"],
+      "js": ["src/content/injector.js"],
+      "css": ["src/content/content.css"],
+      "run_at": "document_start",
+      "all_frames": true
+    }
+  ],
+  "action": {
+    "default_popup": "src/popup/popup.html",
+    "default_icon": {
+      "16": "icons/icon16.svg",
+      "48": "icons/icon48.svg",
+      "128": "icons/icon128.svg"
+    }
+  },
+  "options_page": "src/options/options.html",
+  "icons": {
+    "16": "icons/icon16.svg",
+    "48": "icons/icon48.svg",
+    "128": "icons/icon128.svg"
+  },
+  "web_accessible_resources": [
+    {
+      "resources": ["src/content/content.js"],
+      "matches": ["https://claude.ai/*"]
+    }
+  ]
+}