v3.7.0: roll back from v4, remove internal player — v3 is already complete

Version rolled back from v4.x to v3.7.0. After extensive work on v4's
internal streaming engine, smart queue, DASH pipeline, and media controls,
we realized v3 was already feature-complete. Adding more big features
only made maintenance increasingly difficult and the developer's life
miserable. Stripped back to what works: external player only, cleaner
codebase, sustainable long-term.

- Remove just_audio, audio_service, audio_session and entire internal
  playback engine (smart queue, notification, shuffle/repeat, prefetch)
- Remove PlaybackItem model, MiniPlayerBar widget, notification drawables
- Remove playerMode setting (external-only now)
- Migrate MainActivity from AudioServiceFragmentActivity to
  FlutterFragmentActivity
- Migrate Qobuz to MusicDL API
- Update changelog with v3.7.0 rollback explanation

lib/models/playback_item.dart

@@ -1,91 +0,0 @@
-import 'package:spotiflac_android/models/track.dart';
-
-class PlaybackItem {
-  final String id;
-  final String title;
-  final String artist;
-  final String album;
-  final String coverUrl;
-  final String sourceUri;
-  final bool isLocal;
-  final String service;
-  final int durationMs;
-
-  // Stream quality metadata
-  final String format;
-  final int bitDepth;
-  final int sampleRate;
-  final int bitrate;
-
-  // Original track reference for queue operations
-  final Track? track;
-
-  const PlaybackItem({
-    required this.id,
-    required this.title,
-    required this.artist,
-    this.album = '',
-    this.coverUrl = '',
-    required this.sourceUri,
-    this.isLocal = false,
-    this.service = '',
-    this.durationMs = 0,
-    this.format = '',
-    this.bitDepth = 0,
-    this.sampleRate = 0,
-    this.bitrate = 0,
-    this.track,
-  });
-
-  PlaybackItem copyWith({
-    String? sourceUri,
-    String? service,
-    String? format,
-    int? bitDepth,
-    int? sampleRate,
-    int? bitrate,
-  }) {
-    return PlaybackItem(
-      id: id,
-      title: title,
-      artist: artist,
-      album: album,
-      coverUrl: coverUrl,
-      sourceUri: sourceUri ?? this.sourceUri,
-      isLocal: isLocal,
-      service: service ?? this.service,
-      durationMs: durationMs,
-      format: format ?? this.format,
-      bitDepth: bitDepth ?? this.bitDepth,
-      sampleRate: sampleRate ?? this.sampleRate,
-      bitrate: bitrate ?? this.bitrate,
-      track: track,
-    );
-  }
-
-  /// Human-readable quality label for UI display
-  String get qualityLabel {
-    final parts = <String>[];
-
-    if (format.isNotEmpty) {
-      parts.add(format.toUpperCase());
-    }
-
-    if (bitDepth > 0 && sampleRate > 0) {
-      final srKhz = sampleRate >= 1000
-          ? '${(sampleRate / 1000).toStringAsFixed(sampleRate % 1000 == 0 ? 0 : 1)}kHz'
-          : '${sampleRate}Hz';
-      parts.add('$bitDepth-bit / $srKhz');
-    } else if (bitrate > 0) {
-      parts.add('${bitrate}kbps');
-    }
-
-    return parts.join(' ');
-  }
-
-  /// Whether this item has cover art that is a local file path
-  bool get hasLocalCover {
-    if (coverUrl.isEmpty) return false;
-    return !coverUrl.startsWith('http://') && !coverUrl.startsWith('https://');
-  }
-}

lib/models/settings.dart

@@ -11,9 +11,6 @@ class AppSettings {
   final String storageMode; // 'app' or 'saf'
   final String downloadTreeUri; // SAF persistable tree URI
   final bool autoFallback;
-  final bool autoSkipUnavailableTracks;
-  final String playerMode; // 'internal' or 'external'
-  final bool smartQueueEnabled; // Enable smart curated autoplay queue
   final bool embedMetadata; // Master switch for metadata/cover/lyrics embedding
   final bool embedLyrics;
   final bool maxQualityCover;
@@ -92,9 +89,6 @@ class AppSettings {
     this.storageMode = 'app',
     this.downloadTreeUri = '',
     this.autoFallback = true,
-    this.autoSkipUnavailableTracks = true,
-    this.playerMode = 'internal',
-    this.smartQueueEnabled = true,
     this.embedMetadata = true,
     this.embedLyrics = true,
     this.maxQualityCover = true,
@@ -160,10 +154,7 @@ class AppSettings {
     String? downloadDirectory,
     String? storageMode,
     String? downloadTreeUri,
-    bool? autoFallback,
-    bool? autoSkipUnavailableTracks,
-    String? playerMode,
-    bool? smartQueueEnabled,
+     bool? autoFallback,
     bool? embedMetadata,
     bool? embedLyrics,
     bool? maxQualityCover,
@@ -223,10 +214,6 @@ class AppSettings {
       storageMode: storageMode ?? this.storageMode,
       downloadTreeUri: downloadTreeUri ?? this.downloadTreeUri,
       autoFallback: autoFallback ?? this.autoFallback,
-      autoSkipUnavailableTracks:
-          autoSkipUnavailableTracks ?? this.autoSkipUnavailableTracks,
-      playerMode: playerMode ?? this.playerMode,
-      smartQueueEnabled: smartQueueEnabled ?? this.smartQueueEnabled,
       embedMetadata: embedMetadata ?? this.embedMetadata,
       embedLyrics: embedLyrics ?? this.embedLyrics,
       maxQualityCover: maxQualityCover ?? this.maxQualityCover,

lib/models/settings.g.dart

@@ -14,9 +14,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   storageMode: json['storageMode'] as String? ?? 'app',
   downloadTreeUri: json['downloadTreeUri'] as String? ?? '',
   autoFallback: json['autoFallback'] as bool? ?? true,
-  autoSkipUnavailableTracks: json['autoSkipUnavailableTracks'] as bool? ?? true,
-  playerMode: json['playerMode'] as String? ?? 'internal',
-  smartQueueEnabled: json['smartQueueEnabled'] as bool? ?? true,
   embedMetadata: json['embedMetadata'] as bool? ?? true,
   embedLyrics: json['embedLyrics'] as bool? ?? true,
   maxQualityCover: json['maxQualityCover'] as bool? ?? true,
@@ -93,9 +90,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'storageMode': instance.storageMode,
   'downloadTreeUri': instance.downloadTreeUri,
   'autoFallback': instance.autoFallback,
-  'autoSkipUnavailableTracks': instance.autoSkipUnavailableTracks,
-  'playerMode': instance.playerMode,
-  'smartQueueEnabled': instance.smartQueueEnabled,
   'embedMetadata': instance.embedMetadata,
   'embedLyrics': instance.embedLyrics,
   'maxQualityCover': instance.maxQualityCover,