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/services/platform_bridge.dart

@@ -20,51 +20,6 @@ class PlatformBridge {
     return jsonDecode(result as String) as Map<String, dynamic>;
   }
 
-  static Future<Map<String, dynamic>> getSpotifyMetadata(String url) async {
-    _log.d('getSpotifyMetadata: $url');
-    final result = await _channel.invokeMethod('getSpotifyMetadata', {
-      'url': url,
-    });
-    return jsonDecode(result as String) as Map<String, dynamic>;
-  }
-
-  static Future<Map<String, dynamic>> searchSpotify(
-    String query, {
-    int limit = 10,
-  }) async {
-    _log.d('searchSpotify: "$query" (limit: $limit)');
-    final result = await _channel.invokeMethod('searchSpotify', {
-      'query': query,
-      'limit': limit,
-    });
-    return jsonDecode(result as String) as Map<String, dynamic>;
-  }
-
-  static Future<Map<String, dynamic>> searchSpotifyAll(
-    String query, {
-    int trackLimit = 15,
-    int artistLimit = 3,
-  }) async {
-    _log.d('searchSpotifyAll: "$query"');
-    final result = await _channel.invokeMethod('searchSpotifyAll', {
-      'query': query,
-      'track_limit': trackLimit,
-      'artist_limit': artistLimit,
-    });
-    return jsonDecode(result as String) as Map<String, dynamic>;
-  }
-
-  static Future<Map<String, dynamic>> getSpotifyRelatedArtists(
-    String artistId, {
-    int limit = 12,
-  }) async {
-    final result = await _channel.invokeMethod('getSpotifyRelatedArtists', {
-      'artist_id': artistId,
-      'limit': limit,
-    });
-    return jsonDecode(result as String) as Map<String, dynamic>;
-  }
-
   static Future<Map<String, dynamic>> checkAvailability(
     String spotifyId,
     String isrc,
@@ -517,21 +472,6 @@ class PlatformBridge {
     return result as bool;
   }
 
-  static Future<void> setSpotifyCredentials(
-    String clientId,
-    String clientSecret,
-  ) async {
-    await _channel.invokeMethod('setSpotifyCredentials', {
-      'client_id': clientId,
-      'client_secret': clientSecret,
-    });
-  }
-
-  static Future<bool> hasSpotifyCredentials() async {
-    final result = await _channel.invokeMethod('hasSpotifyCredentials');
-    return result as bool;
-  }
-
   static Future<void> preWarmTrackCache(
     List<Map<String, String>> tracks,
   ) async {
@@ -1313,5 +1253,4 @@ class PlatformBridge {
     });
     return jsonDecode(result as String) as Map<String, dynamic>;
   }
-
 }