v0.2.4: stop export loop on Ctrl+C instead of flooding failures

- Add IsCancelled() to Bridge interface
- Check bridge.IsCancelled() before each export in serial/parallel/backupTree
- Parallel workers mark remaining slots as 'cancelled' instead of exporting
- Add photos_request_is_cancelled to ObjC and C stub

internal/photos/bridge.go

@@ -13,6 +13,7 @@ type Bridge interface {
 	ExportPreview(assetID, outputDir string, targetSize, index int) (ExportResult, error)
 	ExportOriginal(assetID, outputDir string, index int) (ExportResult, error)
 	Cancel()
+	IsCancelled() bool
 }
 
 func ParseAlbumsJSON(jsonStr string) ([]Album, error) {