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/src/lib/cache.ts

@@ -0,0 +1,80 @@
+import { UrlCheckResult } from '../types';
+
+const CACHE_TTL_MS = 5 * 60 * 1000;
+const API_TIMEOUT_MS = 500;
+const MAX_CACHE_SIZE = 5000;
+
+export class UrlCache {
+  private cache: Map<string, { result: UrlCheckResult; expiresAt: number }> = new Map();
+
+  async get(url: string): Promise<UrlCheckResult | null> {
+    const normalized = this.normalizeUrl(url);
+    const entry = this.cache.get(normalized);
+
+    if (!entry) return null;
+
+    if (Date.now() > entry.expiresAt) {
+      this.cache.delete(normalized);
+      return null;
+    }
+
+    return { ...entry.result, cached: true };
+  }
+
+  async set(url: string, result: UrlCheckResult): Promise<void> {
+    const normalized = this.normalizeUrl(url);
+
+    if (this.cache.size >= MAX_CACHE_SIZE) {
+      const firstKey = this.cache.keys().next().value;
+      if (firstKey) this.cache.delete(firstKey);
+    }
+
+    this.cache.set(normalized, {
+      result,
+      expiresAt: Date.now() + CACHE_TTL_MS,
+    });
+  }
+
+  async persistToStorage(): Promise<void> {
+    const entries: Record<string, { result: UrlCheckResult; expiresAt: number }> = {};
+    for (const [key, value] of this.cache.entries()) {
+      entries[key] = value;
+    }
+    await chrome.storage.local.set({ urlCache: entries });
+  }
+
+  async loadFromStorage(): Promise<void> {
+    const data = await chrome.storage.local.get('urlCache');
+    if (data.urlCache) {
+      const now = Date.now();
+      for (const [key, entry] of Object.entries(data.urlCache)) {
+        if (now <= entry.expiresAt) {
+          this.cache.set(key, entry);
+        }
+      }
+    }
+  }
+
+  getStats(): { size: number; max: number } {
+    return { size: this.cache.size, max: MAX_CACHE_SIZE };
+  }
+
+  clear(): void {
+    this.cache.clear();
+  }
+
+  private normalizeUrl(url: string): string {
+    try {
+      const u = new URL(url);
+      u.hash = '';
+      u.search = '';
+      return u.toString();
+    } catch {
+      return url.toLowerCase();
+    }
+  }
+}
+
+export const urlCache = new UrlCache();
+export const CACHE_TTL = CACHE_TTL_MS;
+export const API_TIMEOUT = API_TIMEOUT_MS;