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/esbuild.config.mjs

@@ -0,0 +1,31 @@
+import * as esbuild from "esbuild";
+
+const isWatch = process.argv.includes("--watch");
+
+const buildOptions = {
+  entryPoints: [
+    "src/background/service-worker.ts",
+    "src/popup/popup.ts",
+    "src/options/options.ts",
+  ],
+  bundle: true,
+  outdir: "dist",
+  format: "esm",
+  target: "chrome120",
+  sourcemap: true,
+  minify: !isWatch,
+  // Force CJS resolution for libsodium (ESM entry has broken sibling import)
+  alias: {
+    "libsodium-wrappers-sumo":
+      "./node_modules/libsodium-wrappers-sumo/dist/modules-sumo/libsodium-wrappers.js",
+  },
+};
+
+if (isWatch) {
+  const ctx = await esbuild.context(buildOptions);
+  await ctx.watch();
+  console.log("Watching for changes...");
+} else {
+  await esbuild.build(buildOptions);
+  console.log("Build complete.");
+}