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>

server.js

@@ -132,8 +132,10 @@ app.use(express.static(path.join(__dirname, 'public'), {
 }));
 
 // ─── Auth API ─────────────────────────────────────────────────
+const APP_VERSION = require('./package.json').version;
+
 app.get('/api/config', (_req, res) => {
-  res.json({ signupsOpen: config.signupsOpen });
+  res.json({ signupsOpen: config.signupsOpen, version: APP_VERSION });
 });
 
 // Validate the stored auth token; the client uses this at startup to detect a