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/vite.config.ts

@@ -0,0 +1,32 @@
+import { defineConfig } from 'vite';
+import { resolve } from 'path';
+
+export default defineConfig(({ mode }) => {
+  const isFirefox = mode === 'firefox';
+  const targetDir = isFirefox ? 'dist/firefox' : 'dist/chrome';
+
+  return {
+    root: '.',
+    build: {
+      outDir: targetDir,
+      emptyOutDir: true,
+      rollupOptions: {
+        input: {
+          background: resolve(__dirname, 'src/background/index.ts'),
+          content: resolve(__dirname, 'src/content/index.ts'),
+          popup: resolve(__dirname, 'src/popup/popup.ts'),
+          options: resolve(__dirname, 'src/options/options.ts'),
+        },
+        output: {
+          entryFileNames: '[name].js',
+        },
+      },
+      copyPublicDir: true,
+    },
+    resolve: {
+      alias: {
+        '@': resolve(__dirname, 'src'),
+      },
+    },
+  };
+});