A note from the forge button may carry a screenshot of the page, only when the sender ticks consent #4820

Intent: #4758's optional screenshot, with consent.

Done:

• [ ] the panel has an unticked "Include a picture of this page" box. Only when it is ticked is a screenshot of the visible page taken in the browser (no new dependency; say in the pull how it is taken) and sent with the note
• [ ] the server caps the image (2 MB, PNG or JPEG only, checked by its bytes), stores it under FORGE_APP_DATA, and the issue shows it to those who can read the issue
• [ ] tests: unticked sends no image; an oversized or non-image upload is refused; the image is not readable by someone who cannot read the issue