🎨 Palette: Improve accessibility of radio button groups

[benpiper]

• 💡 What: Replaced the generic <div> and <label> wrapping the "Background Removal Method" radio buttons with semantic <fieldset> and <legend> tags, preserving the original styling via inline CSS.
• 🎯 Why: Screen readers often struggle to associate a generic label with a group of radio buttons. Using <fieldset> and <legend> provides explicit structural context, ensuring users understand what the radio buttons are for.
• 📸 Before/After: Visual appearance remains identical. HTML structure is semantically correct.
• ♿ Accessibility: Improved screen reader announcements by explicitly grouping related form controls.

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PR created automatically by Jules for task 12036626772158324296 started by @benpiper

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