refactor: remove built-in Spotify API provider, use Deezer as sole default

- Remove all Spotify credential management (client ID/secret, secure storage)
- Remove Spotify platform channel handlers from MainActivity
- Remove exported Go functions: GetSpotifyMetadata, SearchSpotify,
  SearchSpotifyAll, GetSpotifyRelatedArtists, SetSpotifyAPICredentials
- Simplify GetSpotifyMetadataWithDeezerFallback to SpotFetch-only path
- Remove Spotify built-in fallback in ReEnrichFile search pipeline
- Always return false from HasSpotifyCredentials; getCredentials always errors
- Default metadataProviderPriority is now ['deezer'] only
- Sanitize provider priority list to strip 'spotify' entries on load/save
- Add migration v5 to clear saved Spotify credentials from existing installs
- Remove Spotify source chip and credentials UI from options settings page
- Remove metadataSource param from search() — always uses Deezer
- spotify-web extension remains supported via the extension provider system

lib/models/settings.dart

@@ -104,7 +104,7 @@ class AppSettings {
     this.askQualityBeforeDownload = true,
     this.spotifyClientId = '',
     this.spotifyClientSecret = '',
-    this.useCustomSpotifyCredentials = true,
+    this.useCustomSpotifyCredentials = false,
     this.metadataSource = 'deezer',
     this.enableLogging = false,
     this.useExtensionProviders = true,
@@ -149,7 +149,7 @@ class AppSettings {
     String? downloadDirectory,
     String? storageMode,
     String? downloadTreeUri,
-     bool? autoFallback,
+    bool? autoFallback,
     bool? embedMetadata,
     bool? embedLyrics,
     bool? maxQualityCover,