fix(flutter): library Artists tab 404 + Albums tab 3-up grid

Artists tab 404: client called /api/library/artists, but the server
mounts the artists list at /api/artists (handleListArtists). Albums
sit at /api/library/albums for historical reasons — the paths aren't
symmetric. Switch listArtists() to /api/artists with sort=alpha.

Albums tab grid: matched the responsive 3-up layout we built for
artist detail. LayoutBuilder computes cellW from available width;
AlbumCard sized to the cell with titleMaxLines: 2; mainAxisExtent
matches actual content height (cover + 2-line title + artist line +
fudge). No more wide-aspect cells with empty space below the card.

Also wired `extra: ref` on the artists/albums grids and the Liked
tab so detail-screen nav hydration kicks in here too — taps from
library screens get the same instant header that home tiles do.

flutter_client/lib/api/endpoints/library_lists.dart

@@ -12,9 +12,16 @@ class LibraryListsApi {
   final Dio _dio;
 
   Future<Paged<ArtistRef>> listArtists({int limit = 50, int offset = 0}) async {
+    // Server mounts the artists list at /api/artists (handleListArtists),
+    // not /api/library/artists. Albums use /api/library/albums for
+    // historical reasons; the paths aren't symmetric.
     final r = await _dio.get<Map<String, dynamic>>(
-      '/api/library/artists',
-      queryParameters: {'limit': limit, 'offset': offset},
+      '/api/artists',
+      queryParameters: {
+        'limit': limit,
+        'offset': offset,
+        'sort': 'alpha',
+      },
     );
     return Paged.fromJson(r.data ?? const {}, ArtistRef.fromJson);
   }