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/style.css

@@ -1040,7 +1040,12 @@ input:focus { border-color: var(--gold); }
   font-size: .72rem;
   color: rgba(255,255,255,.35);
   letter-spacing: .5px;
+  cursor: pointer;
+  user-select: none;
+  -webkit-tap-highlight-color: transparent;
 }
+.app-version:hover  { color: rgba(255,255,255,.6); text-decoration: underline; }
+.app-version:active { color: var(--gold); }
 
 /* ── Round-by-round breakdown ───── */
 .round-breakdown-wrap { margin: 6px 0 2px; }