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/rules/phishing-rules.json

@@ -0,0 +1,58 @@
+[
+  {
+    "id": 1,
+    "priority": 1,
+    "action": { "type": "BLOCK" },
+    "condition": {
+      "urlFilter": "*://*login-secure-portal*/*",
+      "resourceTypes": ["main_frame"]
+    }
+  },
+  {
+    "id": 2,
+    "priority": 1,
+    "action": { "type": "BLOCK" },
+    "condition": {
+      "urlFilter": "*://*account-verify-now*/*",
+      "resourceTypes": ["main_frame"]
+    }
+  },
+  {
+    "id": 3,
+    "priority": 1,
+    "action": { "type": "BLOCK" },
+    "condition": {
+      "urlFilter": "*://*secure-auth-signin*/*",
+      "resourceTypes": ["main_frame"]
+    }
+  },
+  {
+    "id": 4,
+    "priority": 1,
+    "action": { "type": "BLOCK" },
+    "condition": {
+      "urlFilter": "*://*wallet-connect-verify*/*",
+      "resourceTypes": ["main_frame"]
+    }
+  },
+  {
+    "id": 5,
+    "priority": 2,
+    "action": { "type": "REDIRECT", "redirect": { "urlFilter": "chrome-extension://__MSG_@@extension_id__/popup.html" } },
+    "condition": {
+      "urlFilter": "*://*.tk/*",
+      "resourceTypes": ["main_frame"],
+      "domainMatches": ["*.tk"]
+    }
+  },
+  {
+    "id": 6,
+    "priority": 2,
+    "action": { "type": "REDIRECT", "redirect": { "urlFilter": "chrome-extension://__MSG_@@extension_id__/popup.html" } },
+    "condition": {
+      "urlFilter": "*://*.xyz/*",
+      "resourceTypes": ["main_frame"],
+      "domainMatches": ["*.xyz"]
+    }
+  }
+]