feat: add home feed provider setting, fix Qobuz cover URL propagation

- Add homeFeedProvider field to AppSettings with picker UI in extensions page
- Update explore_provider to respect user's home feed provider preference
- Add normalizeCoverReference() and normalizeRemoteHttpUrl() to filter
  invalid cover URLs (no scheme, no host, protocol-relative)
- Apply cover URL normalization across all screens and providers to
  prevent 'no host specified in URI' errors from Qobuz
- Propagate CoverURL from QobuzDownloadResult through Go backend so
  cover art is available even when request metadata is incomplete

lib/models/settings.dart

@@ -34,6 +34,7 @@ class AppSettings {
   final bool enableLogging;
   final bool useExtensionProviders;
   final String? searchProvider;
+  final String? homeFeedProvider;
   final bool separateSingles;
   final String albumFolderStructure;
   final bool showExtensionStore;
@@ -113,6 +114,7 @@ class AppSettings {
     this.enableLogging = false,
     this.useExtensionProviders = true,
     this.searchProvider,
+    this.homeFeedProvider,
     this.separateSingles = false,
     this.albumFolderStructure = 'artist_album',
     this.showExtensionStore = true,
@@ -179,6 +181,8 @@ class AppSettings {
     bool? useExtensionProviders,
     String? searchProvider,
     bool clearSearchProvider = false,
+    String? homeFeedProvider,
+    bool clearHomeFeedProvider = false,
     bool? separateSingles,
     String? albumFolderStructure,
     bool? showExtensionStore,
@@ -244,6 +248,9 @@ class AppSettings {
       searchProvider: clearSearchProvider
           ? null
           : (searchProvider ?? this.searchProvider),
+      homeFeedProvider: clearHomeFeedProvider
+          ? null
+          : (homeFeedProvider ?? this.homeFeedProvider),
       separateSingles: separateSingles ?? this.separateSingles,
       albumFolderStructure: albumFolderStructure ?? this.albumFolderStructure,
       showExtensionStore: showExtensionStore ?? this.showExtensionStore,

lib/models/settings.g.dart

@@ -39,6 +39,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   enableLogging: json['enableLogging'] as bool? ?? false,
   useExtensionProviders: json['useExtensionProviders'] as bool? ?? true,
   searchProvider: json['searchProvider'] as String?,
+  homeFeedProvider: json['homeFeedProvider'] as String?,
   separateSingles: json['separateSingles'] as bool? ?? false,
   albumFolderStructure:
       json['albumFolderStructure'] as String? ?? 'artist_album',
@@ -117,6 +118,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'enableLogging': instance.enableLogging,
   'useExtensionProviders': instance.useExtensionProviders,
   'searchProvider': instance.searchProvider,
+  'homeFeedProvider': instance.homeFeedProvider,
   'separateSingles': instance.separateSingles,
   'albumFolderStructure': instance.albumFolderStructure,
   'showExtensionStore': instance.showExtensionStore,