🎨 Palette: [UX improvement] Improve accessibility of Background Removal Method options

[benpiper]

• What: Replaced the generic div and label elements surrounding the "Background Removal Method" radio buttons with semantic <fieldset> and <legend> tags, using inline CSS to maintain the visual design.
• Why: Generic structural tags for grouped radio buttons deprive screen reader users of structural context. <fieldset> and <legend> explicitly associate the group label with its constituent radio options, significantly improving the experience for screen reader users navigating forms.
• Before/After: The visual representation remains identical, but the underlying DOM semantics provide vastly improved structure for assistive technologies.
• Accessibility: Screen readers now announce the overarching "Background Removal Method" context when users focus on any of the individual radio button choices within the group.

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PR created automatically by Jules for task 1450676373808790796 started by @benpiper

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