feat: SW update banner — notify instead of force-reload

- SW activate now posts SW_UPDATED message to all clients instead of
  calling client.navigate() (which was disruptive and unreliable)
- injectPWA() calls reg.update() on every app open to eagerly check
  for a new SW rather than waiting for the 24h browser interval
- showUpdateBanner() renders a persistent accent-bordered toast with
  version tag, a "Neu laden" button and a dismiss X
- Banner sits above player bar, repositions on mobile above bottom-nav

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.js

@@ -1329,9 +1329,38 @@ async function loadArtists(){
 
 // ─── PWA ───
 function injectPWA(){
-  if ('serviceWorker' in navigator) {
-    navigator.serviceWorker.register('sw.js').catch(() => {});
-  }
+  if (!('serviceWorker' in navigator)) return;
+  navigator.serviceWorker.register('sw.js').then(reg => {
+    // Force-check for SW update every time the app opens
+    reg.update().catch(() => {});
+    // New SW found while page is open (e.g. user left tab open overnight)
+    reg.addEventListener('updatefound', () => {
+      const sw = reg.installing;
+      if (!sw) return;
+      sw.addEventListener('statechange', () => {
+        // 'installed' + existing controller = update ready but waiting for skipWaiting
+        // With skipWaiting() this fires briefly; the SW_UPDATED message handles the banner
+      });
+    });
+  }).catch(() => {});
+
+  // SW posts SW_UPDATED after it activates and claims this client
+  navigator.serviceWorker.addEventListener('message', e => {
+    if (e.data?.type === 'SW_UPDATED') showUpdateBanner(e.data.version);
+  });
+}
+
+function showUpdateBanner(version) {
+  if (document.getElementById('update-banner')) return;
+  const b = document.createElement('div');
+  b.id = 'update-banner';
+  b.className = 'update-banner';
+  b.innerHTML = `
+    <span class="update-banner-text">✦ Update verfügbar${version ? ' <span style="opacity:.55;font-size:10px">('+version+')</span>' : ''}</span>
+    <button class="update-banner-btn" onclick="window.location.reload()">Neu laden</button>
+    <button class="update-banner-close" onclick="document.getElementById('update-banner').remove()">✕</button>
+  `;
+  document.body.appendChild(b);
 }