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/index.html

@@ -91,6 +91,7 @@
 
     <p id="lobby-error" class="error-msg"></p>
   </div>
+  <div id="app-version" class="app-version"></div>
 </div>
 
 <!-- ══════════════ WAITING ROOM ══════════════ -->