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/public/icons/icon128.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128">
+  <defs>
+    <linearGradient id="shieldGrad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#3b82f6;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#1e40af;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  <path d="M64 8 L112 32 L112 72 C112 100 64 120 64 120 C64 120 16 100 16 72 L16 32 Z" fill="url(#shieldGrad)"/>
+  <path d="M52 68 L60 76 L78 56" stroke="white" stroke-width="8" fill="none" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>