🎨 Palette: Add fieldset and legend to radio button group

[benpiper]

💡 What: Replaced a generic div and label with a semantic <fieldset> and <legend> for the background removal method radio buttons.
🎯 Why: Improves context for screen reader users by properly grouping radio buttons so they know what the options relate to.
📸 Before/After: Visuals remain unchanged due to inline CSS border: none; padding: 0; margin: 0;.
♿ Accessibility: Added structural context for screen readers navigating the radio group.

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PR created automatically by Jules for task 1038378757848141575 started by @benpiper

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