client: realtime socket and shared UI components

Add Socket.IO client wrapper (auto-reconnect, auth token), and
reusable components: Layout (nav bar), Toast notifications, and
Countdown timer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/realtime/socket.ts

@@ -0,0 +1,20 @@
+import { io, type Socket } from 'socket.io-client';
+import { API_URL } from '../config';
+
+let socket: Socket | null = null;
+
+/** Connette il socket realtime (path /ws) autenticandosi con il JWT. */
+export function connectSocket(token: string): Socket {
+  disconnectSocket();
+  socket = io(API_URL, { path: '/ws', auth: { token } });
+  return socket;
+}
+
+export function getSocket(): Socket | null {
+  return socket;
+}
+
+export function disconnectSocket(): void {
+  socket?.disconnect();
+  socket = null;
+}