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/smime/enroll/route.ts

@@ -15,12 +15,17 @@ import { NextResponse } from 'next/server';
 import { readStalwartAuthContext } from '@/lib/stalwart/auth-context';
 import { fetchJmapSession, postJmap, rebaseApiUrl } from '@/lib/stalwart/jmap-api';
 import { CaError, getCaProvider } from '@/lib/smime-ca';
+import { isFeatureEnabledServer } from '@/lib/admin/feature-gate';
 
 export const runtime = 'nodejs';
 
 const MAX_CSR_BYTES = 8 * 1024;
 
 export async function POST(request: Request) {
+  if (!isFeatureEnabledServer('smimeEnabled')) {
+    return NextResponse.json({ error: 'Feature disabled' }, { status: 403 });
+  }
+
   const provider = getCaProvider();
   if (!provider) {
     return NextResponse.json(