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/theme/app_theme.dart

@@ -3,7 +3,6 @@ import 'package:flutter/services.dart';
 import 'package:spotiflac_android/models/theme_settings.dart';
 
 class AppTheme {
-  /// Default seed color (Spotify green)
   static const Color defaultSeedColor = Color(kDefaultSeedColor);
 
   static ThemeData light({ColorScheme? dynamicScheme, Color? seedColor}) {
@@ -87,12 +86,10 @@ class AppTheme {
       fontWeight: FontWeight.w500,
     ),
     systemOverlayStyle: SystemUiOverlayStyle(
-      // Status bar
       statusBarColor: Colors.transparent,
       statusBarIconBrightness: scheme.brightness == Brightness.dark
           ? Brightness.light
           : Brightness.dark,
-      // System navigation bar — match the in-app NavigationBar color
       systemNavigationBarColor: isAmoled
           ? Colors.black
           : scheme.surfaceContainer,