feat(flutter/library): artist + album detail screens

Headers carry placeholder play buttons (wired in Task 18) + room for
LikeButton (Task 16). Artist screen has a 2-column album grid; album
screen has a track list with mm:ss durations. Routes :id-parameterized
under the shell so the PlayerBar persists.

flutter_client/lib/library/album_detail_screen.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../theme/theme_extension.dart';
+import 'library_providers.dart';
+import 'widgets/track_row.dart';
+
+class AlbumDetailScreen extends ConsumerWidget {
+  const AlbumDetailScreen({required this.id, super.key});
+  final String id;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    return Scaffold(
+      appBar: AppBar(),
+      backgroundColor: fs.obsidian,
+      body: ref.watch(albumProvider(id)).when(
+            error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
+            loading: () => const Center(child: CircularProgressIndicator()),
+            data: (r) => ListView(children: [
+              Padding(
+                padding: const EdgeInsets.all(16),
+                child: Row(children: [
+                  Container(width: 96, height: 96, color: fs.slate),
+                  const SizedBox(width: 16),
+                  Expanded(child: Column(
+                    crossAxisAlignment: CrossAxisAlignment.start,
+                    children: [
+                      Text(r.album.title, style: TextStyle(
+                        color: fs.parchment,
+                        fontFamily: fs.display.fontFamily,
+                        fontSize: 22,
+                      )),
+                      Text(r.album.artistName, style: TextStyle(color: fs.ash)),
+                    ],
+                  )),
+                  Container(
+                    width: 48, height: 48,
+                    decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),
+                    child: IconButton(
+                      icon: Icon(Icons.play_arrow, color: fs.parchment),
+                      onPressed: () { /* play wired in Task 18 */ },
+                    ),
+                  ),
+                  // LikeButton goes here in Task 16.
+                ]),
+              ),
+              for (final t in r.tracks)
+                TrackRow(track: t, onTap: () { /* play wired in Task 18 */ }),
+            ]),
+          ),
+    );
+  }
+}