fix(flutter): keep artist avatars round in the Library grid

ArtistCard hardcoded its avatar at Container(width: 124, height:
124) inside a ClipOval. In the Library Artists 3-column grid the
cell is narrower than the card's nominal 140dp width — on a typical
phone the cell is ~109dp, the padded inner content area ~93dp. The
parent constrained the Container's width to ~93dp but the explicit
height stayed 124dp, so ClipOval clipped a 93×124 rectangle and
the avatar rendered as a vertical ellipse.

Fix mirrors AlbumCard's pattern: ArtistCard takes an optional
`width` parameter (default 140 for horizontal carousels) and
derives coverSize = width - 16, so the Container is always square.
ArtistsTab now uses LayoutBuilder to compute cell width and passes
it through, same as AlbumsTab. Avatar stays a true circle at any
cell width.

mainAxisExtent on the grid replaces the previous fixed
childAspectRatio so cell height tracks cellW + name line, with
slack matching AlbumsTab's overflow guard.

flutter_client/lib/library/widgets/artist_card.dart

@@ -12,15 +12,27 @@ import '../library_providers.dart';
 import 'play_circle_button.dart';
 
 class ArtistCard extends ConsumerWidget {
-  const ArtistCard({required this.artist, required this.onTap, super.key});
+  const ArtistCard({
+    required this.artist,
+    required this.onTap,
+    this.width = 140,
+    super.key,
+  });
   final ArtistRef artist;
   final VoidCallback onTap;
 
+  /// Outer width of the card. Avatar is a circle of width-16 (8dp
+  /// horizontal padding either side). Default suits horizontal lists;
+  /// grids should pass the cell width so the avatar shrinks
+  /// proportionally and stays a true circle.
+  final double width;
+
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final coverSize = width - 16;
     return SizedBox(
-      width: 140,
+      width: width,
       child: Material(
         color: Colors.transparent,
         child: InkWell(
@@ -29,15 +41,20 @@ class ArtistCard extends ConsumerWidget {
             padding: const EdgeInsets.symmetric(horizontal: 8),
             child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
               // Stack: circular avatar + overlaid play button at bottom-right.
-              // The avatar is a circle (ClipOval) — bottom-right of its
-              // bounding box still places the button inside the visible area
-              // because the button is small relative to the radius.
+              // Avatar size derives from the [width] parameter so the
+              // Library Artists 3-column grid can pass its cell width
+              // (~109dp on typical phones) and the avatar stays a
+              // perfect circle. Previous hardcoded 124×124 got squeezed
+              // horizontally in the grid (cell narrower than 124+16
+              // padding) but kept its 124dp height — ClipOval produced
+              // a visible vertical ellipse. Mirrors AlbumCard's width-
+              // parameter convention.
               Stack(
                 children: [
                   ClipOval(
                     child: Container(
-                      width: 124,
-                      height: 124,
+                      width: coverSize,
+                      height: coverSize,
                       color: fs.slate,
                       child: artist.coverUrl.isEmpty
                           ? SvgPicture.asset('assets/svg/album-fallback.svg',