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/App.tsx

@@ -545,7 +545,7 @@ export default function App() {
           ) : page === "playlists" ? (
             <Playlists canWrite={meQuery.data!.can_write} />
           ) : page === "notifications" ? (
-            <NotificationsPanel filters={filters} setFilters={setFilters} setPage={setPage} onFocusChannel={focusChannel} />
+            <NotificationsPanel filters={filters} setFilters={setFilters} setPage={setPage} onFocusChannel={focusChannel} meId={meQuery.data!.id} isDemo={meQuery.data!.is_demo} />
           ) : page === "settings" ? (
             <SettingsPanel
               me={meQuery.data!}