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

browser-ext/public/manifest.json

@@ -0,0 +1,44 @@
+{
+  "manifest_version": 3,
+  "name": "ShieldAI",
+  "version": "0.1.0",
+  "description": "AI-powered spam call, SMS, and phishing protection",
+  "permissions": [
+    "activeTab",
+    "storage",
+    "declarativeNetRequest",
+    "notifications"
+  ],
+  "host_permissions": [
+    "https://api.shieldai.com/*",
+    "https://*.shieldai.com/*"
+  ],
+  "background": {
+    "service_worker": "background.js",
+    "type": "module"
+  },
+  "content_scripts": [
+    {
+      "matches": ["<all_urls>"],
+      "js": ["content.js"],
+      "run_at": "document_end"
+    }
+  ],
+  "action": {
+    "default_popup": "src/popup/popup.html",
+    "default_icon": {
+      "16": "icons/icon-16.png",
+      "48": "icons/icon-48.png",
+      "128": "icons/icon-128.png"
+    }
+  },
+  "options_ui": {
+    "page": "src/options/options.html",
+    "open_in_tab": true
+  },
+  "icons": {
+    "16": "icons/icon-16.png",
+    "48": "icons/icon-48.png",
+    "128": "icons/icon-128.png"
+  }
+}