Add tap-to-update button on version footer to force-refresh cached assets

Phones stuck on stale service-worker caches (esp. iOS home-screen PWAs) could
show the new version string while still running old app.js/css. Tapping the
footer now unregisters the service worker, clears all caches, and reloads to
pull the latest build.

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

public/app.js

@@ -1700,20 +1700,40 @@ async function validateAuth() {
   updateAuthBar();
 }
 
-// Show the running app version in the lobby footer (served from /api/config)
+// Show the running app version in the lobby footer (served from /api/config).
+// Tapping it force-updates: clears the service-worker caches and reloads, so a
+// phone stuck on stale cached assets can pull the latest build.
 async function loadAppVersion() {
   try {
     const d = await fetch('/api/config').then(r => r.json());
-    if (d.version) $('app-version').textContent = `v${d.version}`;
+    if (d.version) $('app-version').textContent = `v${d.version} · tap to update`;
   } catch { /* ignore */ }
 }
 
+async function forceUpdate() {
+  const el = $('app-version');
+  if (el) el.textContent = 'Updating…';
+  try {
+    if ('serviceWorker' in navigator) {
+      const regs = await navigator.serviceWorker.getRegistrations();
+      await Promise.all(regs.map(r => r.unregister()));
+    }
+    if (window.caches) {
+      const keys = await caches.keys();
+      await Promise.all(keys.map(k => caches.delete(k)));
+    }
+  } catch { /* ignore */ }
+  // SW gone and caches cleared — a plain reload now refetches everything fresh
+  location.reload();
+}
+
 // ─── Boot ─────────────────────────────────────────────────────
 document.addEventListener('DOMContentLoaded', async () => {
   initLobby();
   wireGameEvents();
   applyBarBottom();
   loadAppVersion();
+  $('app-version').addEventListener('click', forceUpdate);
 
   // Drop any stale/corrupted auth token before we use it to connect
   await validateAuth();