feat: propagate download cancel to extension HTTP requests and fix SAF filename extension mismatch

- Bind cancel context to all extension HTTP calls (fetch, httpGet, httpPost,
  httpRequest, fileDownload, authExchangeCodeWithPKCE) so in-flight requests
  are aborted when user cancels a download
- Make initDownloadCancel idempotent: return existing context if entry already
  exists and preserve pre-cancelled state
- Force SAF output filename to match actual file extension when extension
  returns a different format than requested (e.g. FLAC requested but M4A produced)
- Map ALAC/AAC quality to .m4a instead of falling through to default .flac

lib/providers/download_queue_provider.dart

@@ -2378,6 +2378,7 @@ class DownloadQueueNotifier extends Notifier<DownloadQueueState> {
       return '.m4a';
     }
     final q = quality.toLowerCase();
+    if (q == 'alac' || q.startsWith('aac')) return '.m4a';
     if (q.startsWith('opus')) return '.opus';
     if (q.startsWith('mp3')) return '.mp3';
     return '.flac';