feat(browser-ext): move browser extension to browser-ext/ and update API client to tRPC

- Create browser-ext/ with full extension code (MV3 manifest, background
  service worker, content script, popup, options page)
- Add tRPC API client that communicates with unified monolith endpoints
- Implement cache, settings, and phishing detection utilities
- Create extension tRPC router in web app (getAuthStatus, linkDevice,
  reportPhishing)
- Configure Vite build with manifest V3 support
- Write unit tests for cache, phishing detector, and API client
- All 20 tests passing, TypeScript lint clean

package.json

@@ -9,8 +9,11 @@
   "scripts": {
     "dev": "pnpm --filter web dev",
     "build": "pnpm --filter web build",
+    "build:ext": "pnpm --filter @shieldai/browser-ext build",
     "test": "pnpm --filter web test",
+    "test:ext": "pnpm --filter @shieldai/browser-ext test",
     "lint": "pnpm --filter web lint",
+    "lint:ext": "pnpm --filter @shieldai/browser-ext lint",
     "db:migrate": "pnpm --filter web db:migrate",
     "db:seed": "pnpm --filter web db:seed"
   },