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/account-store.ts

@@ -1,5 +1,6 @@
 import { create } from 'zustand';
-import { persist } from 'zustand/middleware';
+import { persist, createJSONStorage } from 'zustand/middleware';
+import { encryptedStorage } from '@/stores/encrypted-storage';
 import { generateAccountId, generateAvatarColor, getMaxAccounts } from '@/lib/account-utils';
 
 export interface AccountEntry {
@@ -218,6 +219,7 @@ export const useAccountStore = create<AccountState>()(
     }),
     {
       name: 'account-registry',
+      storage: createJSONStorage(() => encryptedStorage),
       partialize: (state) => ({
         accounts: state.accounts,
         activeAccountId: state.activeAccountId,