refactor: extract shared mixins and helpers, drop dead abstractions

Introduces shared seams for paginated views, D-pad reorder, media control
routing, async singletons and the device method channel, then points the
open-coded copies at them.

Also removes unused models and duplicated provider/server plumbing, folds
the twice-implemented artifact store in the server, and factors the
repeated Flutter toolchain prologue in CI into a composite action.

test/widgets/video_settings_sheet_test.dart

@@ -10,6 +10,7 @@ import 'package:plezy/mpv/player/player_streams.dart';
 import 'package:plezy/screens/settings/subtitle_styling_screen.dart';
 import 'package:plezy/services/sleep_timer_service.dart';
 import 'package:plezy/services/settings_service.dart';
+import 'package:plezy/widgets/video_controls/models/track_controls_state.dart';
 import 'package:plezy/widgets/video_controls/sheets/video_settings_sheet.dart';
 
 import '../test_helpers/prefs.dart';
@@ -147,10 +148,8 @@ Future<void> _pumpSheet(
           height: 700,
           child: VideoSettingsSheet(
             player: player ?? _FakeSettingsPlayer(),
-            audioSyncOffset: 0,
-            subtitleSyncOffset: 0,
-            canControl: canControl,
             supportsHdrControl: supportsHdrControl,
+            trackControlsState: TrackControlsState(canControl: canControl),
           ),
         ),
       ),