feat: Phase 3+4 — security hardening + polish + offline + Electron push

Phase 3 (security):
- P3.1: Feature gate server-side enforcement (403 on disabled features)
- P3.2: Unified auth error interceptor (401→logout)
- P3.3: Store-level state isolation via StoreSnapshot contract
  (added message-list-tabs + task stores to snapshot/restore cycle)
- P3.4: Push event bus extraction — email-store no longer imports
  calendar/contact/filter/file stores directly
- P1.3: Auth localStorage AES-GCM encryption via custom Zustand adapter

Phase 4 (polish):
- P4.1: Offline write queue — pending operations in localStorage,
  auto-retry on reconnect, offline-queue-indicator banner
- P4.2: Identity spoofing — fromOverrideEmail domain validation
- P4.3: WebSocket push for Electron via main-process IPC bridge
  (ws package with Authorization headers)

app/api/plugins/route.ts

@@ -3,6 +3,7 @@ import { getPluginRegistry, getThemeRegistry } from '@/lib/admin/plugin-registry
 import { listDevPlugins } from '@/lib/admin/plugin-dev';
 import { configManager } from '@/lib/admin/config-manager';
 import { logger } from '@/lib/logger';
+import { isFeatureEnabledServer } from '@/lib/admin/feature-gate';
 
 /**
  * GET /api/plugins - Public endpoint for clients to discover server-managed plugins & themes
@@ -11,6 +12,10 @@ import { logger } from '@/lib/logger';
  * No admin auth required - this is how regular users receive plugins/themes.
  */
 export async function GET() {
+  if (!isFeatureEnabledServer('pluginsEnabled')) {
+    return NextResponse.json({ error: 'Feature disabled' }, { status: 403 });
+  }
+
   try {
     await configManager.ensureLoaded();
     const policy = configManager.getPolicy();