🎨 Palette: Semantic grouping for radio buttons

[benpiper]

What: Replaced the generic div wrapper around the background removal radio group with a semantic <fieldset> and <legend>.
Why: Radio buttons grouped in standard divs lack context for screen reader users, making it difficult to understand what the options represent.
Before/After: Visuals remain unchanged due to styling resets, but the underlying structure is updated.
Accessibility: Enhances screen reader compatibility by explicitly linking the radio buttons to the "Background Removal Method" legend.

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PR created automatically by Jules for task 12707265538285940046 started by @benpiper

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