fix(flutter): null-coalesce PlaylistTrack.streamUrl to TrackRef.streamUrl

PlaylistTrack.streamUrl is String? (nullable when track is unavailable
post-delete); TrackRef.streamUrl is required String. flutter analyze
caught the mismatch. Coalesce to empty string for unavailable rows —
they're already filtered out by the trackId != null check earlier in
the loop, so this branch is effectively unreachable but keeps the
type-checker happy.

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

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -116,7 +116,7 @@ class PlaylistCard extends ConsumerWidget {
         artistId: t.artistId ?? '',
         artistName: t.artistName,
         durationSec: t.durationSec,
-        streamUrl: t.streamUrl,
+        streamUrl: t.streamUrl ?? '',
       ));
     }
     if (refs.isEmpty) return;