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/screens/tutorial_screen.dart

@@ -707,10 +707,6 @@ class _TutorialPage extends StatelessWidget {
     final contentGap = (56 * scale) + ((textScale - 1) * 10);
     final bottomGap = (32 * scale).clamp(20.0, 32.0);
 
-    // Parallax effect logic (simplified for StatelessWidget)
-    // In a real advanced implementation we'd pass the Controller's listenable
-    // But for now, let's use entrance animations based on currentIndex == index
-
     final isActive = currentIndex == index;
 
     return SingleChildScrollView(