feat(library): playlists filter view and configurable default view

Add a fourth Playlists chip/page to the Library filter pager, rendered
as an album-style grid of playlist cells (search filters by name, page
provider short-circuits since playlists come from libraryCollections).
Add a defaultLibraryView setting (last used / all / albums / singles /
playlists) in Library settings, applied on startup and whenever the
shell switches back to the Library tab.

lib/models/settings.dart

@@ -34,6 +34,10 @@ class AppSettings {
   final bool filterContributingArtistsInAlbumArtist;
   final String historyViewMode;
   final String historyFilterMode;
+
+  /// Library view opened when switching to the Library tab:
+  /// 'last' (keep last used), 'all', 'albums', 'singles', or 'playlists'.
+  final String defaultLibraryView;
   final bool askQualityBeforeDownload;
   final bool enableLogging;
   final bool useExtensionProviders;
@@ -132,6 +136,7 @@ class AppSettings {
     this.filterContributingArtistsInAlbumArtist = false,
     this.historyViewMode = 'grid',
     this.historyFilterMode = 'all',
+    this.defaultLibraryView = 'last',
     this.askQualityBeforeDownload = true,
     this.enableLogging = false,
     this.useExtensionProviders = true,
@@ -202,6 +207,7 @@ class AppSettings {
     bool? filterContributingArtistsInAlbumArtist,
     String? historyViewMode,
     String? historyFilterMode,
+    String? defaultLibraryView,
     bool? askQualityBeforeDownload,
     bool? enableLogging,
     bool? useExtensionProviders,
@@ -279,6 +285,7 @@ class AppSettings {
           this.filterContributingArtistsInAlbumArtist,
       historyViewMode: historyViewMode ?? this.historyViewMode,
       historyFilterMode: historyFilterMode ?? this.historyFilterMode,
+      defaultLibraryView: defaultLibraryView ?? this.defaultLibraryView,
       askQualityBeforeDownload:
           askQualityBeforeDownload ?? this.askQualityBeforeDownload,
       enableLogging: enableLogging ?? this.enableLogging,

lib/models/settings.g.dart

@@ -33,6 +33,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['filterContributingArtistsInAlbumArtist'] as bool? ?? false,
   historyViewMode: json['historyViewMode'] as String? ?? 'grid',
   historyFilterMode: json['historyFilterMode'] as String? ?? 'all',
+  defaultLibraryView: json['defaultLibraryView'] as String? ?? 'last',
   askQualityBeforeDownload: json['askQualityBeforeDownload'] as bool? ?? true,
   enableLogging: json['enableLogging'] as bool? ?? false,
   useExtensionProviders: json['useExtensionProviders'] as bool? ?? true,
@@ -122,6 +123,7 @@ Map<String, dynamic> _$AppSettingsToJson(
       instance.filterContributingArtistsInAlbumArtist,
   'historyViewMode': instance.historyViewMode,
   'historyFilterMode': instance.historyFilterMode,
+  'defaultLibraryView': instance.defaultLibraryView,
   'askQualityBeforeDownload': instance.askQualityBeforeDownload,
   'enableLogging': instance.enableLogging,
   'useExtensionProviders': instance.useExtensionProviders,