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)

package.json

@@ -84,6 +84,7 @@
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "webcrypto-liner": "^1.4.3",
+    "ws": "^8.21.3",
     "zustand": "^5.0.12"
   },
   "optionalDependencies": {