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)

lib/push-event-bus.ts

@@ -0,0 +1,38 @@
+import type { IJMAPClient } from '@/lib/jmap/client-interface';
+
+type JmapTypeHandler = (client: IJMAPClient, accountChanges: Record<string, string>) => Promise<void>;
+
+const handlers = new Map<string, JmapTypeHandler[]>();
+
+export function registerPushHandler(jmapType: string, handler: JmapTypeHandler): () => void {
+  const list = handlers.get(jmapType) ?? [];
+  list.push(handler);
+  handlers.set(jmapType, list);
+  return () => {
+    const current = handlers.get(jmapType);
+    if (!current) return;
+    const idx = current.indexOf(handler);
+    if (idx >= 0) current.splice(idx, 1);
+    if (current.length === 0) handlers.delete(jmapType);
+  };
+}
+
+export async function dispatchPushEvent(
+  client: IJMAPClient,
+  changed: Record<string, Record<string, string>>,
+  accountId: string,
+): Promise<void> {
+  const accountChanges = changed[accountId];
+
+  for (const [jmapType, typeHandlers] of handlers) {
+    if (accountChanges?.[jmapType]) {
+      for (const handler of typeHandlers) {
+        try {
+          await handler(client, accountChanges);
+        } catch (error) {
+          console.error(`Push handler for ${jmapType} failed:`, error);
+        }
+      }
+    }
+  }
+}