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>

styles.css

@@ -704,6 +704,34 @@ nav{flex:1;overflow-y:auto;padding:4px 10px}
 .mobile-overlay{display:none;position:fixed;inset:0;background:rgba(0,0,0,.6);z-index:140}
 .mobile-overlay.active{display:block}
 
+/* ── UPDATE BANNER ── */
+.update-banner{
+  position:fixed;bottom:calc(var(--player-h) + 20px);left:50%;
+  transform:translateX(-50%);
+  display:flex;align-items:center;gap:10px;
+  background:var(--surface);border:1px solid var(--accent);
+  border-radius:12px;padding:10px 14px;
+  box-shadow:0 8px 32px rgba(0,0,0,.55),0 0 0 1px rgba(232,160,32,.15);
+  z-index:202;white-space:nowrap;
+  animation:toastIn .3s ease;
+}
+.update-banner-text{font-size:13px;font-weight:600;color:var(--text)}
+.update-banner-btn{
+  background:var(--accent);color:#000;border:none;
+  padding:6px 14px;border-radius:7px;
+  font-size:12px;font-weight:700;cursor:pointer;font-family:var(--font);
+  transition:opacity .15s;
+}
+.update-banner-btn:hover{opacity:.85}
+.update-banner-close{
+  background:none;border:none;color:var(--text3);
+  cursor:pointer;font-size:15px;padding:0 2px;line-height:1;
+}
+.update-banner-close:hover{color:var(--text)}
+@media(max-width:768px){
+  .update-banner{bottom:calc(var(--player-h) + 80px + 12px)}
+}
+
 /* ── SEARCH page ── */
 .search-section{margin-bottom:24px}
 .search-section-title{font-size:11px;font-weight:700;color:var(--text2);text-transform:uppercase;letter-spacing:.5px;margin-bottom:8px}