feat: lyrics provider extensions, configurable lyrics cascade, and iOS method channel parity

Add lyrics_provider as a new extension type alongside metadata_provider and
download_provider. Extensions implementing fetchLyrics() are called before
built-in providers, giving user-installed extensions highest priority.

Built-in lyrics cascade is now configurable from Download Settings:
  - Reorderable provider list (LRCLIB, Musixmatch, Netease, Apple Music, QQ Music)
  - Per-provider options: Netease translation/romanization, Apple/QQ multi-person
    word-by-word speaker tags, Musixmatch language code
  - Provider order and options synced to Go backend on app start and on change

Go backend changes:
  - New lyrics_provider manifest type with validation (extension_manifest.go)
  - ExtensionProviderWrapper.FetchLyrics() with Goja JS bridge (extension_providers.go)
  - Configurable SetLyricsProviderOrder/GetLyricsProviderOrder cascade (lyrics.go)
  - LyricsFetchOptions struct for per-provider settings (lyrics.go)
  - Extracted tryLRCLIB() helper, randomized LRCLIB User-Agent (lyrics.go)
  - Refactored msToLRCTimestamp to separate msToLRCTimestampInline (lyrics.go)
  - New provider source files: lyrics_apple.go, lyrics_musixmatch.go,
    lyrics_netease.go, lyrics_qqmusic.go
  - JSON export functions for lyrics settings (exports.go)
  - hasLyricsProvider field in extension manager JSON output

Platform channels:
  - Android (MainActivity.kt): setLyricsProviders, getLyricsProviders,
    getAvailableLyricsProviders, setLyricsFetchOptions, getLyricsFetchOptions
  - iOS (AppDelegate.swift): same 5 method channel handlers for iOS parity

Flutter side:
  - Extension model: hasLyricsProvider field + Lyrics Provider capability badge
  - Settings model: lyricsProviders, lyricsIncludeTranslationNetease,
    lyricsIncludeRomanizationNetease, lyricsMultiPersonWordByWord,
    musixmatchLanguage fields with generated serialization
  - Settings provider: setters + _syncLyricsSettingsToBackend()
  - Download settings UI: provider picker, toggle switches, language picker
  - Platform bridge: lyrics provider/options methods

Docs: lyrics provider extension documentation in site/docs.html
CHANGELOG: updated with lyrics provider and search feature entries

lib/models/settings.dart

@@ -56,6 +56,18 @@ class AppSettings {
   final bool
   hasCompletedTutorial; // Track if user has completed the app tutorial
 
+  // Lyrics Provider Settings
+  final List<String>
+  lyricsProviders; // Ordered list of enabled lyrics provider IDs
+  final bool
+  lyricsIncludeTranslationNetease; // Append translated lyrics (Netease)
+  final bool
+  lyricsIncludeRomanizationNetease; // Append romanized lyrics (Netease)
+  final bool
+  lyricsMultiPersonWordByWord; // Enable v1/v2 + [bg:] tags for Apple/QQ syllable lyrics
+  final String
+  musixmatchLanguage; // Optional ISO language code for Musixmatch localized lyrics
+
   const AppSettings({
     this.defaultService = 'tidal',
     this.audioQuality = 'LOSSLESS',
@@ -100,6 +112,12 @@ class AppSettings {
     this.localLibraryShowDuplicates = true,
     // Tutorial default
     this.hasCompletedTutorial = false,
+    // Lyrics providers default order
+    this.lyricsProviders = const ['lrclib', 'musixmatch', 'netease', 'apple_music', 'qqmusic'],
+    this.lyricsIncludeTranslationNetease = false,
+    this.lyricsIncludeRomanizationNetease = false,
+    this.lyricsMultiPersonWordByWord = true,
+    this.musixmatchLanguage = '',
   });
 
   AppSettings copyWith({
@@ -147,6 +165,12 @@ class AppSettings {
     bool? localLibraryShowDuplicates,
     // Tutorial
     bool? hasCompletedTutorial,
+    // Lyrics providers
+    List<String>? lyricsProviders,
+    bool? lyricsIncludeTranslationNetease,
+    bool? lyricsIncludeRomanizationNetease,
+    bool? lyricsMultiPersonWordByWord,
+    String? musixmatchLanguage,
   }) {
     return AppSettings(
       defaultService: defaultService ?? this.defaultService,
@@ -202,6 +226,15 @@ class AppSettings {
           localLibraryShowDuplicates ?? this.localLibraryShowDuplicates,
       // Tutorial
       hasCompletedTutorial: hasCompletedTutorial ?? this.hasCompletedTutorial,
+      // Lyrics providers
+      lyricsProviders: lyricsProviders ?? this.lyricsProviders,
+      lyricsIncludeTranslationNetease:
+          lyricsIncludeTranslationNetease ?? this.lyricsIncludeTranslationNetease,
+      lyricsIncludeRomanizationNetease:
+          lyricsIncludeRomanizationNetease ?? this.lyricsIncludeRomanizationNetease,
+      lyricsMultiPersonWordByWord:
+          lyricsMultiPersonWordByWord ?? this.lyricsMultiPersonWordByWord,
+      musixmatchLanguage: musixmatchLanguage ?? this.musixmatchLanguage,
     );
   }
 

lib/models/settings.g.dart

@@ -53,50 +53,68 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   localLibraryShowDuplicates:
       json['localLibraryShowDuplicates'] as bool? ?? true,
   hasCompletedTutorial: json['hasCompletedTutorial'] as bool? ?? false,
+  lyricsProviders:
+      (json['lyricsProviders'] as List<dynamic>?)
+          ?.map((e) => e as String)
+          .toList() ??
+      const ['lrclib', 'musixmatch', 'netease', 'apple_music', 'qqmusic'],
+  lyricsIncludeTranslationNetease:
+      json['lyricsIncludeTranslationNetease'] as bool? ?? false,
+  lyricsIncludeRomanizationNetease:
+      json['lyricsIncludeRomanizationNetease'] as bool? ?? false,
+  lyricsMultiPersonWordByWord:
+      json['lyricsMultiPersonWordByWord'] as bool? ?? true,
+  musixmatchLanguage: json['musixmatchLanguage'] as String? ?? '',
 );
 
-Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
-    <String, dynamic>{
-      'defaultService': instance.defaultService,
-      'audioQuality': instance.audioQuality,
-      'filenameFormat': instance.filenameFormat,
-      'downloadDirectory': instance.downloadDirectory,
-      'storageMode': instance.storageMode,
-      'downloadTreeUri': instance.downloadTreeUri,
-      'autoFallback': instance.autoFallback,
-      'embedLyrics': instance.embedLyrics,
-      'maxQualityCover': instance.maxQualityCover,
-      'isFirstLaunch': instance.isFirstLaunch,
-      'concurrentDownloads': instance.concurrentDownloads,
-      'checkForUpdates': instance.checkForUpdates,
-      'updateChannel': instance.updateChannel,
-      'hasSearchedBefore': instance.hasSearchedBefore,
-      'folderOrganization': instance.folderOrganization,
-      'useAlbumArtistForFolders': instance.useAlbumArtistForFolders,
-      'usePrimaryArtistOnly': instance.usePrimaryArtistOnly,
-      'filterContributingArtistsInAlbumArtist':
-          instance.filterContributingArtistsInAlbumArtist,
-      'historyViewMode': instance.historyViewMode,
-      'historyFilterMode': instance.historyFilterMode,
-      'askQualityBeforeDownload': instance.askQualityBeforeDownload,
-      'spotifyClientId': instance.spotifyClientId,
-      'spotifyClientSecret': instance.spotifyClientSecret,
-      'useCustomSpotifyCredentials': instance.useCustomSpotifyCredentials,
-      'metadataSource': instance.metadataSource,
-      'enableLogging': instance.enableLogging,
-      'useExtensionProviders': instance.useExtensionProviders,
-      'searchProvider': instance.searchProvider,
-      'separateSingles': instance.separateSingles,
-      'albumFolderStructure': instance.albumFolderStructure,
-      'showExtensionStore': instance.showExtensionStore,
-      'locale': instance.locale,
-      'lyricsMode': instance.lyricsMode,
-      'tidalHighFormat': instance.tidalHighFormat,
-      'useAllFilesAccess': instance.useAllFilesAccess,
-      'autoExportFailedDownloads': instance.autoExportFailedDownloads,
-      'downloadNetworkMode': instance.downloadNetworkMode,
-      'localLibraryEnabled': instance.localLibraryEnabled,
-      'localLibraryPath': instance.localLibraryPath,
-      'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,
-      'hasCompletedTutorial': instance.hasCompletedTutorial,
-    };
+Map<String, dynamic> _$AppSettingsToJson(
+  AppSettings instance,
+) => <String, dynamic>{
+  'defaultService': instance.defaultService,
+  'audioQuality': instance.audioQuality,
+  'filenameFormat': instance.filenameFormat,
+  'downloadDirectory': instance.downloadDirectory,
+  'storageMode': instance.storageMode,
+  'downloadTreeUri': instance.downloadTreeUri,
+  'autoFallback': instance.autoFallback,
+  'embedLyrics': instance.embedLyrics,
+  'maxQualityCover': instance.maxQualityCover,
+  'isFirstLaunch': instance.isFirstLaunch,
+  'concurrentDownloads': instance.concurrentDownloads,
+  'checkForUpdates': instance.checkForUpdates,
+  'updateChannel': instance.updateChannel,
+  'hasSearchedBefore': instance.hasSearchedBefore,
+  'folderOrganization': instance.folderOrganization,
+  'useAlbumArtistForFolders': instance.useAlbumArtistForFolders,
+  'usePrimaryArtistOnly': instance.usePrimaryArtistOnly,
+  'filterContributingArtistsInAlbumArtist':
+      instance.filterContributingArtistsInAlbumArtist,
+  'historyViewMode': instance.historyViewMode,
+  'historyFilterMode': instance.historyFilterMode,
+  'askQualityBeforeDownload': instance.askQualityBeforeDownload,
+  'spotifyClientId': instance.spotifyClientId,
+  'spotifyClientSecret': instance.spotifyClientSecret,
+  'useCustomSpotifyCredentials': instance.useCustomSpotifyCredentials,
+  'metadataSource': instance.metadataSource,
+  'enableLogging': instance.enableLogging,
+  'useExtensionProviders': instance.useExtensionProviders,
+  'searchProvider': instance.searchProvider,
+  'separateSingles': instance.separateSingles,
+  'albumFolderStructure': instance.albumFolderStructure,
+  'showExtensionStore': instance.showExtensionStore,
+  'locale': instance.locale,
+  'lyricsMode': instance.lyricsMode,
+  'tidalHighFormat': instance.tidalHighFormat,
+  'useAllFilesAccess': instance.useAllFilesAccess,
+  'autoExportFailedDownloads': instance.autoExportFailedDownloads,
+  'downloadNetworkMode': instance.downloadNetworkMode,
+  'localLibraryEnabled': instance.localLibraryEnabled,
+  'localLibraryPath': instance.localLibraryPath,
+  'localLibraryShowDuplicates': instance.localLibraryShowDuplicates,
+  'hasCompletedTutorial': instance.hasCompletedTutorial,
+  'lyricsProviders': instance.lyricsProviders,
+  'lyricsIncludeTranslationNetease': instance.lyricsIncludeTranslationNetease,
+  'lyricsIncludeRomanizationNetease': instance.lyricsIncludeRomanizationNetease,
+  'lyricsMultiPersonWordByWord': instance.lyricsMultiPersonWordByWord,
+  'musixmatchLanguage': instance.musixmatchLanguage,
+};