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)

stores/file-store.ts

@@ -1048,3 +1048,13 @@ export const useFileStore = create<FileState>((set, get) => ({
     }
   },
 }));
+
+import { registerPushHandler } from '@/lib/push-event-bus';
+
+registerPushHandler('FileNode', async (_client) => {
+  const store = useFileStore.getState();
+  const currentParentId = store.currentParentId;
+  store.navigate(currentParentId).catch((err) => {
+    console.error('Failed to refresh files on push:', err);
+  });
+});