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/src/lib/libsodium-wrappers-sumo.d.ts

@@ -0,0 +1,40 @@
+declare module "libsodium-wrappers-sumo" {
+  interface KeyPair {
+    publicKey: Uint8Array;
+    privateKey: Uint8Array;
+    keyType: string;
+  }
+
+  interface Sodium {
+    ready: Promise<void>;
+    crypto_sign_keypair(): KeyPair;
+    crypto_box_keypair(): KeyPair;
+    crypto_scalarmult(privateKey: Uint8Array, publicKey: Uint8Array): Uint8Array;
+    crypto_generichash(hashLength: number, message: Uint8Array): Uint8Array;
+    crypto_aead_xchacha20poly1305_ietf_encrypt(
+      message: Uint8Array,
+      additionalData: Uint8Array | null,
+      nsec: Uint8Array | null,
+      nonce: Uint8Array,
+      key: Uint8Array,
+    ): Uint8Array;
+    crypto_aead_xchacha20poly1305_ietf_decrypt(
+      nsec: Uint8Array | null,
+      ciphertext: Uint8Array,
+      additionalData: Uint8Array | null,
+      nonce: Uint8Array,
+      key: Uint8Array,
+    ): Uint8Array;
+    crypto_aead_xchacha20poly1305_ietf_NPUBBYTES: number;
+    crypto_sign_detached(message: Uint8Array, privateKey: Uint8Array): Uint8Array;
+    crypto_sign_verify_detached(
+      signature: Uint8Array,
+      message: Uint8Array,
+      publicKey: Uint8Array,
+    ): boolean;
+    randombytes_buf(length: number): Uint8Array;
+  }
+
+  const sodium: Sodium;
+  export default sodium;
+}