feat: add auto-scan option for local library

Add a new 'Auto Scan' setting under Local Library with four modes:
off, every app open (10min cooldown), daily, and weekly. The app
uses WidgetsBindingObserver to trigger incremental scans on launch
and when resuming from background, respecting the configured
cooldown based on the last scan timestamp.

lib/models/settings.dart

@@ -59,6 +59,8 @@ class AppSettings {
   localLibraryBookmark; // Base64-encoded iOS security-scoped bookmark
   final bool
   localLibraryShowDuplicates; // Show indicator when searching for existing tracks
+  final String
+  localLibraryAutoScan; // Auto-scan mode: 'off', 'on_open', 'daily', 'weekly'
 
   final bool
   hasCompletedTutorial; // Track if user has completed the app tutorial
@@ -123,6 +125,7 @@ class AppSettings {
     this.localLibraryPath = '',
     this.localLibraryBookmark = '',
     this.localLibraryShowDuplicates = true,
+    this.localLibraryAutoScan = 'off',
     this.hasCompletedTutorial = false,
     this.lyricsProviders = const [
       'lrclib',
@@ -186,6 +189,7 @@ class AppSettings {
     String? localLibraryPath,
     String? localLibraryBookmark,
     bool? localLibraryShowDuplicates,
+    String? localLibraryAutoScan,
     bool? hasCompletedTutorial,
     List<String>? lyricsProviders,
     bool? lyricsIncludeTranslationNetease,
@@ -251,6 +255,8 @@ class AppSettings {
       localLibraryBookmark: localLibraryBookmark ?? this.localLibraryBookmark,
       localLibraryShowDuplicates:
           localLibraryShowDuplicates ?? this.localLibraryShowDuplicates,
+      localLibraryAutoScan:
+          localLibraryAutoScan ?? this.localLibraryAutoScan,
       hasCompletedTutorial: hasCompletedTutorial ?? this.hasCompletedTutorial,
       lyricsProviders: lyricsProviders ?? this.lyricsProviders,
       lyricsIncludeTranslationNetease:

lib/models/settings.g.dart

@@ -57,6 +57,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   localLibraryBookmark: json['localLibraryBookmark'] as String? ?? '',
   localLibraryShowDuplicates:
       json['localLibraryShowDuplicates'] as bool? ?? true,
+  localLibraryAutoScan: json['localLibraryAutoScan'] as String? ?? 'off',
   hasCompletedTutorial: json['hasCompletedTutorial'] as bool? ?? false,
   lyricsProviders:
       (json['lyricsProviders'] as List<dynamic>?)
@@ -129,6 +130,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'localLibraryPath': instance.localLibraryPath,
   'localLibraryBookmark': instance.localLibraryBookmark,
   'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,
+  'localLibraryAutoScan': instance.localLibraryAutoScan,
   'hasCompletedTutorial': instance.hasCompletedTutorial,
   'lyricsProviders': instance.lyricsProviders,
   'lyricsIncludeTranslationNetease': instance.lyricsIncludeTranslationNetease,