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/library/artist_detail_screen.dart

@@ -1,10 +1,8 @@
-import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../api/endpoints/likes.dart';
-import '../cache/metadata_prefetcher.dart';
 import '../likes/like_button.dart';
 import '../models/album.dart';
 import '../models/artist.dart';