Increase robustness of image loading check before sending canvas data.
Now retries up to 5 times with 200ms delays (1 second total) instead
of a single 100ms wait.
This fixes the 'Failed to get confirmation from server' error that
appeared when executing workflows immediately after ComfyUI restart,
before images finished loading from IndexedDB.
Prevents workflow execution failures due to timing issues during
canvas initialization.
66cbcb641b