🎨 Palette: Improve accessibility of background removal method radio group

[benpiper]

💡 What: Replaced the generic <div> and <label> grouping the 'Background Removal Method' radio buttons with semantic <fieldset> and <legend> tags, while preserving the existing visual styling.
🎯 Why: Screen readers often struggle to associate a generic text label with a group of radio buttons.
📸 Before/After: Visual appearance remains identical.
♿ Accessibility: Using <fieldset> and <legend> ensures that assistive technologies correctly announce the group name ('Background Removal Method') when a user focuses on any of the radio buttons within that group.

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PR created automatically by Jules for task 15779664532651117188 started by @benpiper

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