Add a "save to output" option to the image preview node

### Feature Idea

If one decides that an image in the preview of the "preview image" node looks good, it would be nice to have a "save to output" context menu icon that copies it to the output folder. The current "save image" function downloads it to the browser.

### Existing Solutions

- Replace the "preview image" with a "save image" node and re-run the workflow. This is not a good solution if the workflow contains items that are randomized after a run or never cached.
- Download the file and manually save it into the output folder. The solution is only usable if ComfyUI runs on the same computer as the browser and the filename is not adjusted for the current numbering in the output folder.