fix: permission error message and Android 13+ storage permission

- Fixed download showing 'Song not found' when actually permission error
- Added permission error type detection in Go backend
- Android 13+ now requests both MANAGE_EXTERNAL_STORAGE and READ_MEDIA_AUDIO
- MANAGE_EXTERNAL_STORAGE opens Settings (system-level)
- READ_MEDIA_AUDIO shows dialog (app-level, resets on clear data)
- Proper permission check before showing 'granted' status

lib/models/download_item.dart

@@ -19,6 +19,7 @@ enum DownloadErrorType {
   notFound,    // Track not found on any service
   rateLimit,   // Rate limited by service
   network,     // Network/connection error
+  permission,  // File/folder permission error
 }
 
 @JsonSerializable()
@@ -88,6 +89,8 @@ class DownloadItem {
         return 'Rate limit reached, try again later';
       case DownloadErrorType.network:
         return 'Connection failed, check your internet';
+      case DownloadErrorType.permission:
+        return 'Cannot write to folder, check storage permission';
       default:
         return error ?? 'An error occurred';
     }

lib/models/download_item.g.dart

@@ -51,4 +51,5 @@ const _$DownloadErrorTypeEnumMap = {
   DownloadErrorType.notFound: 'notFound',
   DownloadErrorType.rateLimit: 'rateLimit',
   DownloadErrorType.network: 'network',
+  DownloadErrorType.permission: 'permission',
 };