refactor: share the toolbar scrim and dedupe playback and download paths

Extracts the repeated toolbar fade into a single ToolbarScrim widget, folds
duplicated request/retry handling in the media server HTTP client, and
collapses the parallel playback-source, download-manager and live TV helper
paths into shared implementations.

lib/services/playback_source_resolver.dart

@@ -1,11 +1,7 @@
 import '../database/app_database.dart';
 import '../media/ids.dart';
 import '../media/media_backend.dart';
-import '../media/media_item.dart';
 import '../media/media_server_client.dart';
-import '../models/audio_quality_preset.dart';
-import '../models/transcode_quality_preset.dart';
-import '../mpv/mpv.dart';
 import 'multi_server_manager.dart';
 import 'playback_context.dart';
 import 'playback_initialization_service.dart';
@@ -17,40 +13,20 @@ class PlaybackSourceResolver {
   const PlaybackSourceResolver({required this.serverManager, required this.database});
 
   /// [preferOffline] overrides the default downloaded-copy preference
-  /// (`offlineLibraryMode || qualityPreset.isOriginal`). Pass false for
-  /// flows that must stay on the server stream, e.g. a transcode restart.
-  ///
-  /// [audioQualityPreset] is the music transcode preset, consulted by the
-  /// backends only for [MediaKind.track] items ([qualityPreset] is
-  /// video-shaped and ignored for tracks).
-  Future<PlaybackContext> resolve({
-    required MediaItem metadata,
-    required int selectedMediaIndex,
-    String? selectedMediaSourceId,
-    String? preferredVersionSignature,
+  /// (`offlineLibraryMode || options.qualityPreset.isOriginal`, so an omitted
+  /// preset keeps it on). Pass false for flows that must stay on the server
+  /// stream, e.g. a transcode restart.
+  Future<PlaybackContext> resolve(
+    PlaybackInitializationOptions options, {
     required bool offlineLibraryMode,
-    required TranscodeQualityPreset qualityPreset,
-    AudioQualityPreset? audioQualityPreset,
-    int? selectedAudioStreamId,
-    SubtitleTrack? preferredSubtitleTrack,
-    String? sessionIdentifier,
-    String? transcodeSessionId,
     bool? preferOffline,
   }) async {
+    final metadata = options.metadata;
     final reportingClient = _playbackClient(serverIdOrNull(metadata.serverId), offlineLibraryMode: offlineLibraryMode);
     final service = PlaybackInitializationService(client: reportingClient, database: database);
     final result = await service.getPlaybackData(
-      metadata: metadata,
-      selectedMediaIndex: selectedMediaIndex,
-      selectedMediaSourceId: selectedMediaSourceId,
-      preferredVersionSignature: preferredVersionSignature,
-      preferOffline: preferOffline ?? (offlineLibraryMode || qualityPreset.isOriginal),
-      qualityPreset: qualityPreset,
-      audioQualityPreset: audioQualityPreset,
-      selectedAudioStreamId: selectedAudioStreamId,
-      preferredSubtitleTrack: preferredSubtitleTrack,
-      sessionIdentifier: sessionIdentifier,
-      transcodeSessionId: transcodeSessionId,
+      options,
+      preferOffline: preferOffline ?? (offlineLibraryMode || options.qualityPreset.isOriginal),
     );
 
     final sourceKind = result.usesLocalMedia
@@ -75,7 +51,7 @@ class PlaybackSourceResolver {
       streamHeaders: _streamHeaders(
         client: reportingClient,
         sourceKind: sourceKind,
-        sessionIdentifier: sessionIdentifier,
+        sessionIdentifier: options.sessionIdentifier,
       ),
     );
   }