fix(flutter): cover URLs resolve against server baseUrl + audio stream URL guard

Cover-art Image.network calls were passing server-relative URLs
(/api/albums/<id>/cover) straight to NetworkImage, which interprets
"no scheme" as file:/// and crashes with "No host specified in URI".
Same root cause regardless of HTTPS or HTTP server.

ServerImage wraps Image.network with a Riverpod read of
serverUrlProvider and prefixes the configured base URL. Absolute URLs
(e.g. discover screen's Lidarr image_urls) pass through unchanged.

Three call sites updated: album_card, artist_card, playlists_list_screen.
discover_screen left as-is — its row.imageUrl is already absolute (Lidarr
returns full URLs from MusicBrainz / Spotify metadata) and it has a
meaningful errorBuilder that ServerImage doesn't expose.

Also adds a defensive check in audio_handler.setQueueFromTracks: if
the constructed stream URL ends up scheme-less, throw a StateError
naming baseUrl + track.streamUrl + track.id instead of letting it
fall through to ExoPlayer which surfaces a confusing "Cleartext HTTP
traffic to 127.0.0.1 not permitted" error (Android's URL parser
defaults a scheme-less URI to localhost). User reported this exact
confusing error against an HTTPS prod server; the better message
will pinpoint where the empty baseUrl comes from on next reproduction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/playlists/playlists_list_screen.dart

@@ -4,6 +4,7 @@ import 'package:go_router/go_router.dart';
 
 import '../models/playlist.dart';
 import '../shared/widgets/main_app_bar_actions.dart';
+import '../shared/widgets/server_image.dart';
 import '../theme/theme_extension.dart';
 import 'playlists_provider.dart';
 
@@ -81,7 +82,7 @@ class _PlaylistTile extends StatelessWidget {
               color: fs.slate,
               child: playlist.coverUrl.isEmpty
                   ? Icon(Icons.queue_music, color: fs.ash)
-                  : Image.network(playlist.coverUrl, fit: BoxFit.cover),
+                  : ServerImage(url: playlist.coverUrl, fit: BoxFit.cover),
             ),
           ),
           const SizedBox(width: 12),