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/services/platform_bridge.dart

@@ -142,6 +142,10 @@ class PlatformBridge {
     });
   }
 
+  static Future<void> exitApp() async {
+    await _channel.invokeMethod('exitApp');
+  }
+
   static Future<void> initItemProgress(String itemId) async {
     await _channel.invokeMethod('initItemProgress', {'item_id': itemId});
   }

lib/services/update_checker.dart

@@ -74,6 +74,13 @@ class UpdateChecker {
         return null;
       }
 
+      // Ignore releases from a different major version (e.g. v4.x when we
+      // rolled back to v3.x). Only offer updates within the same major line.
+      if (_majorVersion(latestVersion) != _majorVersion(AppInfo.version)) {
+        _log.i('Skipping update from different major version (current: ${AppInfo.version}, latest: $latestVersion)');
+        return null;
+      }
+
       final body = releaseData['body'] as String? ?? 'No changelog available';
       final htmlUrl = releaseData['html_url'] as String? ?? '${AppInfo.githubUrl}/releases';
       final publishedAt = DateTime.tryParse(releaseData['published_at'] as String? ?? '') ?? DateTime.now();
@@ -118,6 +125,14 @@ class UpdateChecker {
     }
   }
 
+  static int _majorVersion(String version) {
+    try {
+      return int.parse(version.split('-').first.split('.').first);
+    } catch (_) {
+      return -1;
+    }
+  }
+
   static bool _isNewerVersion(String latest, String current) {
     try {
       final latestBase = latest.split('-').first;