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

ios/Runner/AppDelegate.swift

@@ -342,6 +342,15 @@ import Gobackend  // Import Go framework
             if let error = error { throw error }
             return response
             
+        case "rewriteSplitArtistTags":
+            let args = call.arguments as! [String: Any]
+            let filePath = args["file_path"] as! String
+            let artist = args["artist"] as! String
+            let albumArtist = args["album_artist"] as! String
+            let response = GobackendRewriteSplitArtistTagsExport(filePath, artist, albumArtist, &error)
+            if let error = error { throw error }
+            return response
+            
         case "cleanupConnections":
             GobackendCleanupConnections()
             return nil