fix: service selection priority and Amazon fallback-only

- Fix service selection ignored: user's preferred service now takes priority
- Add preferredService parameter to downloadWithExtensions
- Gray out Amazon in service picker (fallback only)
- Clean up unused code in Go backend

lib/main.dart

@@ -43,7 +43,6 @@ class _EagerInitializationState extends ConsumerState<_EagerInitialization> {
   void initState() {
     super.initState();
     _initializeExtensions();
-    // Trigger history provider initialization without subscribing to updates.
     ref.read(downloadHistoryProvider);
   }