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-lock.json

@@ -48,6 +48,7 @@
         "sonner": "^2.0.7",
         "tailwind-merge": "^3.4.0",
         "webcrypto-liner": "^1.4.3",
+        "ws": "^8.21.3",
         "zustand": "^5.0.12"
       },
       "devDependencies": {
@@ -12858,6 +12859,27 @@
       "dev": true,
       "license": "ISC"
     },
+    "node_modules/ws": {
+      "version": "8.21.3",
+      "resolved": "https://registry.npmjs.org/ws/-/ws-8.21.3.tgz",
+      "integrity": "sha512-201TZ/kPWxoPr/OKWjquZR1SWKXcvxdH+e1xrx89b3YbmzLMFCLfnaG1HFIgWzJOEWZ7MvpK++odZufgYR50Rw==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=10.0.0"
+      },
+      "peerDependencies": {
+        "bufferutil": "^4.0.1",
+        "utf-8-validate": ">=5.0.2"
+      },
+      "peerDependenciesMeta": {
+        "bufferutil": {
+          "optional": true
+        },
+        "utf-8-validate": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/xml-name-validator": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/xml-name-validator/-/xml-name-validator-5.0.0.tgz",