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/utils/path_match_keys.dart

@@ -25,9 +25,6 @@ const _audioExtensions = <String>[
 const _maxPathMatchKeyCacheSize = 6000;
 final Map<String, Set<String>> _pathMatchKeyCache = <String, Set<String>>{};
 
-/// Strips a trailing audio extension from [path] if present.
-/// Returns the path without extension, or `null` if no known audio extension
-/// was found.
 String? _stripAudioExtension(String path) {
   final lower = path.toLowerCase();
   for (final ext in _audioExtensions) {
@@ -115,8 +112,6 @@ Set<String> buildPathMatchKeys(String? filePath) {
 
   addNormalized(cleaned);
 
-  // Add extension-stripped variants so that a file converted from one audio
-  // format to another (e.g. Song.flac → Song.opus) still matches.
   final extensionStrippedKeys = <String>{};
   for (final key in keys) {
     final stripped = _stripAudioExtension(key);