fix(flutter): tighter track row + missing artist in mini player + hide redundant artist on artist page

Track row (album detail):
- Vertical padding 10→6 and only render the artist line when non-empty,
  so tracks without a populated artist row don't reserve a blank line.
- Number column 28→22 — visually tighter without making 3-digit
  numbers cramp.

Mini player missing artist:
- Symptom was an empty `artist` field on MediaItem after tapping a
  track in album detail; layout change wasn't the cause. Real cause:
  albumProvider's cold-cache only wrote cachedAlbums + cachedTracks,
  never cachedArtists. The drift JOINs that build the AlbumRef + each
  TrackRef returned null for the artist row, so artistName fell back
  to '' and the audio handler stamped MediaItem.artist=''.
- Cold-cache now collects every distinct (artistId, artistName) pair
  off the album header + each track, inserts them into cachedArtists
  alongside the album/tracks. Subsequent JOINs populate artistName,
  the audio handler stamps it on MediaItem, and the mini player picks
  it up via the existing artistName.isNotEmpty render.
- Existing cached albums won't get artist names until they're
  re-fetched (cache invalidation TBD). Future cold-cache hits will.

Artist detail album grid:
- AlbumCard gains showArtist (default true). Pass false in the artist
  detail grid since the page header already names the artist. cellH
  trimmed by the 16dp artist line.

flutter_client/lib/library/library_providers.dart

@@ -179,7 +179,35 @@ final albumProvider = StreamProvider.family<
               .getAlbum(albumId)
               .timeout(const Duration(seconds: 10));
           debugPrint('albumProvider($albumId): got ${fresh.tracks.length} tracks, writing to drift');
+          // Collect every artist mentioned by the album + its tracks so
+          // the JOINs that drive both the album header and the track
+          // rows have something to bind to. Without this, drift returns
+          // null for the artist row and artistName surfaces empty —
+          // visible as a missing artist line in the mini player and
+          // the row list.
+          final artists = <String, ArtistRef>{};
+          if (fresh.album.artistId.isNotEmpty &&
+              fresh.album.artistName.isNotEmpty) {
+            artists[fresh.album.artistId] = ArtistRef(
+              id: fresh.album.artistId,
+              name: fresh.album.artistName,
+            );
+          }
+          for (final t in fresh.tracks) {
+            if (t.artistId.isNotEmpty &&
+                t.artistName.isNotEmpty &&
+                !artists.containsKey(t.artistId)) {
+              artists[t.artistId] = ArtistRef(
+                id: t.artistId,
+                name: t.artistName,
+              );
+            }
+          }
           await db.batch((b) {
+            if (artists.isNotEmpty) {
+              b.insertAllOnConflictUpdate(
+                  db.cachedArtists, artists.values.map((a) => a.toDrift()).toList());
+            }
             b.insertAllOnConflictUpdate(db.cachedAlbums, [fresh.album.toDrift()]);
             b.insertAllOnConflictUpdate(db.cachedTracks,
                 fresh.tracks.map((t) => t.toDrift()).toList());