feat: Edit Playlist modal — name, description, public toggle, cover upload

- 'Bearbeiten' button in playlist-detail header opens modal
- Subsonic updatePlaylist API: name, comment, public synced
- Cover upload via POST /api/playlist/{id}/image (Navidrome REST, Basic Auth)
- Cover delete via DELETE /api/playlist/{id}/image
- Graceful toast if Navidrome version doesn't support cover upload
- Mobile-responsive layout (stacks cover + fields on small screens)
- Bump SW cache to ns-v31

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

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v30';
+const CACHE = 'ns-v31';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];