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/vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['tests/**/*.test.ts'],
+  },
+});