Add ShieldAI browser extension with phishing & spam detection (FRE-4576)

- Extension package: Manifest V3, background service worker, content scripts
- Phishing detection engine with heuristic analysis (typosquatting, entropy, TLD, brand impersonation)
- Local URL caching layer (Storage API) for <100ms cached lookups
- Popup UI with protection status, stats, and phishing report button
- Options page for settings management (blocked/allowed domains, feature toggles)
- Server-side extension routes: URL check, phishing report, auth, stats, exposure check
- Tier-aware feature gating (Basic/Plus/Premium)
- 25 passing tests for phishing detection heuristics
- Declarative net request rules for known phishing patterns
- DarkWatch integration for credential exposure checks
- Firefox compatibility layer via build modes

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

packages/extension/package.json

@@ -0,0 +1,23 @@
+{
+  "name": "@shieldai/extension",
+  "version": "0.1.0",
+  "private": true,
+  "description": "ShieldAI Browser Extension - Phishing & Spam Protection",
+  "scripts": {
+    "build": "vite build",
+    "build:chrome": "vite build --mode chrome",
+    "build:firefox": "vite build --mode firefox",
+    "dev": "vite build --watch --mode chrome",
+    "test": "vitest run",
+    "lint": "eslint src/"
+  },
+  "dependencies": {
+    "@shieldai/types": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/chrome": "^0.0.268",
+    "vite": "^5.4.0",
+    "typescript": "^5.7.0",
+    "vitest": "^4.1.5"
+  }
+}