feat: add resolve API with SongLink fallback, fix multi-artist tags (#288), and cleanup

Resolve API (api.zarz.moe):
- Refactor songlink.go: Spotify URLs use resolve API, non-Spotify uses SongLink API
- Add SongLink fallback when resolve API fails for Spotify (two-layer resilience)
- Remove dead code: page parser, XOR-obfuscated keys, legacy helpers

Multi-artist tag fix (#288):
- Add RewriteSplitArtistTags() in Go to rewrite ARTIST/ALBUMARTIST as split Vorbis comments
- Wire method channel handler in Android (MainActivity.kt) and iOS (AppDelegate.swift)
- Add PlatformBridge.rewriteSplitArtistTags() in Dart
- Call native FLAC rewriter after FFmpeg embed when split_vorbis mode is active
- Extract deezerTrackArtistDisplay() helper to use Contributors in album/playlist tracks

Code cleanup:
- Remove unused imports, dead code, and redundant comments across Go and Dart
- Fix build: remove stale getQobuzDebugKey() reference in deezer_download.go

lib/providers/download_queue_provider.dart

@@ -20,6 +20,7 @@ import 'package:spotiflac_android/services/history_database.dart';
 import 'package:spotiflac_android/utils/logger.dart';
 import 'package:spotiflac_android/utils/file_access.dart';
 import 'package:spotiflac_android/utils/string_utils.dart';
+import 'package:spotiflac_android/utils/artist_utils.dart';
 
 final _log = AppLogger('DownloadQueue');
 final _historyLog = AppLogger('DownloadHistory');
@@ -3010,6 +3011,22 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
         _log.w('FFmpeg metadata/cover embed failed');
       }
 
+      // After FFmpeg embed, fix split artist tags using native FLAC writer.
+      // FFmpeg deduplicates repeated metadata keys, so multiple ARTIST entries
+      // collapse into one. The Go FLAC writer rewrites them properly.
+      if (settings.artistTagMode == artistTagModeSplitVorbis) {
+        try {
+          await PlatformBridge.rewriteSplitArtistTags(
+            flacPath,
+            track.artistName,
+            albumArtist,
+          );
+          _log.d('Split artist tags rewritten via native FLAC writer');
+        } catch (e) {
+          _log.w('Failed to rewrite split artist tags: $e');
+        }
+      }
+
       if (coverPath != null) {
         try {
           final coverFile = File(coverPath);

lib/providers/store_provider.dart

@@ -146,7 +146,6 @@ class StoreState {
     this.registryUrl = '',
   });
 
-  /// Whether a registry URL has been configured by the user.
   bool get hasRegistryUrl => registryUrl.isNotEmpty;
 
   StoreState copyWith({
@@ -218,7 +217,6 @@ class StoreNotifier extends Notifier<StoreState> {
   Future<void> initialize(String cacheDir) async {
     if (state.isInitialized) return;
 
-    // Load saved registry URL early to avoid UI flash (empty → setup screen)
     final prefs = await SharedPreferences.getInstance();
     final savedUrl = prefs.getString(_registryUrlPrefKey) ?? '';
 
@@ -246,8 +244,6 @@ class StoreNotifier extends Notifier<StoreState> {
     }
   }
 
-  /// Sets the registry URL, saves it, and refreshes the store.
-  /// The Go backend handles URL normalisation (GitHub repo → raw URL, branch detection).
   Future<void> setRegistryUrl(String url) async {
     final trimmed = url.trim();
     if (trimmed.isEmpty) {
@@ -258,10 +254,8 @@ class StoreNotifier extends Notifier<StoreState> {
     state = state.copyWith(isLoading: true, clearError: true);
 
     try {
-      // Go backend resolves GitHub URLs (detects default branch) and validates HTTPS.
       await PlatformBridge.setStoreRegistryUrl(trimmed);
 
-      // Read back the resolved URL (may differ from input after normalisation).
       final resolvedUrl = await PlatformBridge.getStoreRegistryUrl();
 
       final prefs = await SharedPreferences.getInstance();
@@ -280,13 +274,11 @@ class StoreNotifier extends Notifier<StoreState> {
     }
   }
 
-  /// Removes the saved registry URL and fully detaches the repo from backend.
   Future<void> removeRegistryUrl() async {
     try {
       final prefs = await SharedPreferences.getInstance();
       await prefs.remove(_registryUrlPrefKey);
 
-      // Reset the URL in Go backend memory AND clear its cache
       await PlatformBridge.clearStoreRegistryUrl();
 
       state = state.copyWith(