diag(flutter): instrument playTracks + cache appdir; fix CI

CI fixes:
- artist_detail_screen.dart: drop unnecessary foundation import (debugPrint
  comes from material) and unused metadata_prefetcher import.

Playback timing visibility (so we can stop guessing where the lag
lives):
- playTracks now logs serverUrl / token / configure / setQueue /
  play() returned, each stage in milliseconds. The next time you
  tap play, we'll see exactly where the seconds go.
- setQueueFromTracks adds two more measurements: total source-build
  time across all tracks, and setAudioSources duration.

Small concrete win:
- audio_handler caches the application cache dir path on first use
  (already cached in _maybeRegisterStreamCache; now also used in
  _buildAudioSource for the LockCachingAudioSource path). One less
  platform channel hit per track on cache-miss queue builds.

Once we see real numbers we can decide whether the fix is to build
sources lazily (initial source first → play → background-add the
rest), pre-warm the audio handler at app start so playTracks skips
serverUrl + token reads entirely, or something else.

flutter_client/lib/player/audio_handler.dart

@@ -80,12 +80,18 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
 
     debugPrint('audio_handler.setQueueFromTracks: '
         '_baseUrl="$_baseUrl" trackCount=${tracks.length}');
+    final sw = Stopwatch()..start();
     final sources = await Future.wait(tracks.map(_buildAudioSource));
+    debugPrint('audio_handler: built ${sources.length} sources in '
+        '${sw.elapsedMilliseconds}ms');
+    sw.reset();
+    sw.start();
 
     await _player.setAudioSources(
       sources,
       initialIndex: initialIndex,
     );
+    debugPrint('audio_handler: setAudioSources ${sw.elapsedMilliseconds}ms');
 
     // Kick the cover fetch for the initial item — async, doesn't block
     // playback. Subsequent track changes are handled by the
@@ -143,8 +149,8 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     // pin / Download buttons cover the index path; LockCaching handles
     // the network optimization.
     if (mgr != null) {
-      final cacheDir = await getApplicationCacheDirectory();
-      final cacheFile = File('${cacheDir.path}/audio_cache/${t.id}.mp3');
+      _cacheDirPath ??= (await getApplicationCacheDirectory()).path;
+      final cacheFile = File('${_cacheDirPath!}/audio_cache/${t.id}.mp3');
       debugPrint('audio_handler: cache miss for track.id=${t.id}, '
           'using LockCachingAudioSource → ${cacheFile.path}');
       // ignore: experimental_member_use