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/sw.js

@@ -1,5 +1,5 @@
 'use strict';
-const CACHE_NAME = 'shelem-v2';
+const CACHE_NAME = 'shelem-v3';
 
 // ── Generate all 55 card paths ──────────────────────────────────
 const SUITS = ['CLUB', 'DIAMOND', 'HEART', 'SPADE'];