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/lib/messagesSocket.ts

@@ -0,0 +1,78 @@
+// Live message delivery over a WebSocket. The server pushes a {type:"message", message} frame
+// to all of a user's open tabs when a message is stored; subscribers react (e.g. refetch the
+// thread + conversations). Auto-reconnects with backoff; a low-frequency poll elsewhere is the
+// safety net if the socket is down.
+import type { Message } from "./api";
+
+type Handler = (msg: Message) => void;
+
+const handlers = new Set<Handler>();
+let ws: WebSocket | null = null;
+let started = false;
+let backoff = 1000;
+let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+
+function url(): string {
+  const proto = location.protocol === "https:" ? "wss" : "ws";
+  return `${proto}://${location.host}/api/messages/ws`;
+}
+
+function open() {
+  try {
+    ws = new WebSocket(url());
+  } catch {
+    schedule();
+    return;
+  }
+  ws.onopen = () => {
+    backoff = 1000;
+  };
+  ws.onmessage = (ev) => {
+    try {
+      const data = JSON.parse(ev.data);
+      if (data?.type === "message" && data.message) {
+        for (const h of handlers) h(data.message as Message);
+      }
+    } catch {
+      /* ignore malformed frames */
+    }
+  };
+  ws.onclose = () => {
+    ws = null;
+    if (started) schedule();
+  };
+  ws.onerror = () => {
+    ws?.close();
+  };
+}
+
+function schedule() {
+  if (reconnectTimer) return;
+  reconnectTimer = setTimeout(() => {
+    reconnectTimer = null;
+    if (started) open();
+  }, backoff);
+  backoff = Math.min(backoff * 2, 30000);
+}
+
+// Subscribe to live messages. Opens the socket on the first subscriber and closes it when the
+// last one leaves.
+export function onMessage(h: Handler): () => void {
+  handlers.add(h);
+  if (!started) {
+    started = true;
+    open();
+  }
+  return () => {
+    handlers.delete(h);
+    if (handlers.size === 0) {
+      started = false;
+      if (reconnectTimer) {
+        clearTimeout(reconnectTimer);
+        reconnectTimer = null;
+      }
+      ws?.close();
+      ws = null;
+    }
+  };
+}