feat: implement M2 Chrome browser extension

Build the CookieBridge Chrome extension (Manifest V3) with:

- Background service worker: cookie monitoring via chrome.cookies.onChanged,
  WebSocket connection to relay server with auto-reconnect, HTTP polling
  fallback, device registration and pairing flow
- Browser-compatible crypto: libsodium-wrappers-sumo for XChaCha20-Poly1305
  encryption, Ed25519 signing, X25519 key exchange (mirrors server's
  sodium-native API)
- Popup UI: device registration, connection status indicator (gray/blue/
  green/red), cookie/device/sync stats, one-click current site sync,
  whitelist quick-add, device pairing with 6-digit code
- Options page: server URL config, connection mode (auto/WS/polling),
  poll interval slider, auto-sync toggle, domain whitelist/blacklist
  management, paired device list, key export/import, data clearing
- Sync engine: LWW conflict resolution with Lamport clocks (same as
  server), bidirectional cookie sync with all paired peers, echo
  suppression to prevent sync loops
- Badge management: icon color reflects state (gray=not logged in,
  blue=connected, green=syncing with count, red=error)
- Build system: esbuild bundling for Chrome 120+, TypeScript with
  strict mode, clean type checking

Co-Authored-By: Paperclip <noreply@paperclip.ing>

extension/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "cookiebridge-extension",
+  "version": "0.1.0",
+  "private": true,
+  "description": "CookieBridge Chrome Extension — cross-device cookie sync with E2E encryption",
+  "scripts": {
+    "build": "node esbuild.config.mjs",
+    "watch": "node esbuild.config.mjs --watch",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "libsodium-wrappers-sumo": "^0.7.15"
+  },
+  "devDependencies": {
+    "@types/chrome": "^0.0.287",
+    "esbuild": "^0.24.2",
+    "typescript": "^5.9.3"
+  }
+}