Show app version in lobby footer; expose version via /api/config

- Server reads version from package.json and returns it from /api/config
- Client fetches it at boot and renders it in a lobby footer
- Bump service-worker cache to shelem-v3 so precache refreshes

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/app.js

@@ -1700,11 +1700,20 @@ async function validateAuth() {
   updateAuthBar();
 }
 
+// Show the running app version in the lobby footer (served from /api/config)
+async function loadAppVersion() {
+  try {
+    const d = await fetch('/api/config').then(r => r.json());
+    if (d.version) $('app-version').textContent = `v${d.version}`;
+  } catch { /* ignore */ }
+}
+
 // ─── Boot ─────────────────────────────────────────────────────
 document.addEventListener('DOMContentLoaded', async () => {
   initLobby();
   wireGameEvents();
   applyBarBottom();
+  loadAppVersion();
 
   // Drop any stale/corrupted auth token before we use it to connect
   await validateAuth();