[TEST] Remove Background tool

[jbaddley]

Route

/remove-background

Scenarios to test

Upload

• Dropzone is visible on load
• Uploading a PNG with a solid background loads the image
• Uploading a JPEG with a white background loads the image
• The original image is displayed in a preview

Tolerance slider

• A tolerance slider is visible after upload
• The slider has a sensible default (e.g. 40)
• Moving the slider to a higher value removes more background pixels
• Moving the slider to a lower value removes fewer pixels
• The preview updates live (or near-live) as the slider moves
• The slider does not crash at its minimum (0) or maximum value

Background removal output

• The preview shows the image with the background replaced by a transparent/checkerboard pattern
• For a solid-white-background image, the white area is removed at a mid tolerance
• For an image with no solid background, most pixels are preserved at low tolerance

Download

• Clicking "Download PNG" triggers a .png download (PNG is required to preserve transparency)
• The downloaded file contains transparent pixels where the background was (not white or black)
• The download button is visible (not hidden behind another element — regression from earlier bug)

Change image

• A "Change image" or reset button returns the tool to the dropzone state

Acceptance criteria

• Tool removes background from a solid-color test image at default tolerance
• Downloaded PNG file has alpha channel (transparency)
• Download button is always visible on desktop