feat: add local library scanning with duplicate detection

- Add Go backend library scanner for FLAC, M4A, MP3, Opus, OGG files
- Read metadata from file tags (ISRC, track name, artist, album, bit depth, sample rate)
- Fallback to filename parsing when tags unavailable
- Add SQLite database for O(1) duplicate lookups
- Show 'In Library' badge on search results for existing tracks
- Match by ISRC (exact) or track name + artist (fuzzy)
- Add Library Settings page with scan, cleanup, and clear actions
- Add 30+ localization strings for library feature

lib/models/settings.dart

@@ -42,8 +42,13 @@ class AppSettings {
   final String cloudProvider; // 'none', 'webdav', 'sftp', 'gdrive'
   final String cloudServerUrl; // WebDAV/SFTP server URL
   final String cloudUsername; // Server username
-  final String cloudPassword; // Server password (encrypted)
+  final String cloudPassword; // Server password (stored securely)
   final String cloudRemotePath; // Remote folder path (e.g. /Music/SpotiFLAC)
+  
+  // Local Library Settings
+  final bool localLibraryEnabled; // Enable local library scanning
+  final String localLibraryPath; // Path to scan for audio files
+  final bool localLibraryShowDuplicates; // Show indicator when searching for existing tracks
 
   const AppSettings({
     this.defaultService = 'tidal',
@@ -85,6 +90,10 @@ class AppSettings {
     this.cloudUsername = '',
     this.cloudPassword = '',
     this.cloudRemotePath = '/Music/SpotiFLAC',
+    // Local Library defaults
+    this.localLibraryEnabled = false,
+    this.localLibraryPath = '',
+    this.localLibraryShowDuplicates = true,
   });
 
   AppSettings copyWith({
@@ -128,6 +137,10 @@ class AppSettings {
     String? cloudUsername,
     String? cloudPassword,
     String? cloudRemotePath,
+    // Local Library
+    bool? localLibraryEnabled,
+    String? localLibraryPath,
+    bool? localLibraryShowDuplicates,
   }) {
     return AppSettings(
       defaultService: defaultService ?? this.defaultService,
@@ -169,6 +182,10 @@ class AppSettings {
       cloudUsername: cloudUsername ?? this.cloudUsername,
       cloudPassword: cloudPassword ?? this.cloudPassword,
       cloudRemotePath: cloudRemotePath ?? this.cloudRemotePath,
+      // Local Library
+      localLibraryEnabled: localLibraryEnabled ?? this.localLibraryEnabled,
+      localLibraryPath: localLibraryPath ?? this.localLibraryPath,
+      localLibraryShowDuplicates: localLibraryShowDuplicates ?? this.localLibraryShowDuplicates,
     );
   }
 

lib/models/settings.g.dart

@@ -48,6 +48,10 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   cloudUsername: json['cloudUsername'] as String? ?? '',
   cloudPassword: json['cloudPassword'] as String? ?? '',
   cloudRemotePath: json['cloudRemotePath'] as String? ?? '/Music/SpotiFLAC',
+  localLibraryEnabled: json['localLibraryEnabled'] as bool? ?? false,
+  localLibraryPath: json['localLibraryPath'] as String? ?? '',
+  localLibraryShowDuplicates:
+      json['localLibraryShowDuplicates'] as bool? ?? true,
 );
 
 Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
@@ -90,4 +94,7 @@ Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
       'cloudUsername': instance.cloudUsername,
       'cloudPassword': instance.cloudPassword,
       'cloudRemotePath': instance.cloudRemotePath,
+      'localLibraryEnabled': instance.localLibraryEnabled,
+      'localLibraryPath': instance.localLibraryPath,
+      'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,
     };