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>

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v27';
+const CACHE = 'ns-v28';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 
@@ -12,8 +12,8 @@ self.addEventListener('activate', e => {
     caches.keys()
       .then(keys => Promise.all(keys.filter(k => k !== CACHE).map(k => caches.delete(k))))
       .then(() => self.clients.claim())
-      .then(() => self.clients.matchAll({type: 'window', includeUncontrolled: true}))
-      .then(clients => clients.forEach(c => c.navigate(c.url)))
+      .then(() => self.clients.matchAll({ type: 'window', includeUncontrolled: true }))
+      .then(clients => clients.forEach(c => c.postMessage({ type: 'SW_UPDATED', version: CACHE })))
   );
 });