feat(messages): E2EE real-time messaging UI + system welcome

A Messages tab in the notification center (hidden for demo): set up secure
messaging with a passphrase (key generated + wrapped in-browser via WebCrypto,
stored non-extractable per device), unlock on other devices, then chat with
end-to-end encrypted, live-delivered messages. The server-authored Siftlode
welcome is readable before any key setup.

- lib/e2ee.ts: ECDH P-256 + HKDF + AES-GCM, PBKDF2-wrapped key, IndexedDB.
- lib/messagesSocket.ts: WebSocket client with backoff reconnect.
- Messages.tsx: key gate, conversation list with decrypted previews, directory,
  thread with client-side decrypt + encrypt-on-send. Unread feeds the nav badge.
  EN/HU/DE strings.

frontend/src/i18n/locales/de/inbox.json

@@ -2,6 +2,7 @@
   "navLabel": "Benachrichtigungen",
   "title": "Benachrichtigungen",
   "subtitle": "Neuigkeiten aus deiner Bibliothek und vom System.",
+  "tabInbox": "Benachrichtigungen",
   "empty": "Alles erledigt.",
   "markAllRead": "Alle als gelesen markieren",
   "clearAll": "Alle löschen",