🎨 Palette: [UX improvement] Improve accessibility of background removal radio buttons

[benpiper]

💡 What: Replaced the generic div wrapper and label for the "Background Removal Method" radio group with a semantic

and in web_app/templates/index.html.
🎯 Why: Radio buttons grouped within a generic div lack context for screen reader users. Using a fieldset and legend ensures the group name is announced correctly when navigating through the options.
📸 Before/After: Visual appearance is completely unchanged due to the application of inline reset styles on the fieldset and legend.
♿ Accessibility: Improved form semantics and structural context for assistive technologies interacting with the background removal options.

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PR created automatically by Jules for task 17614347046125874536 started by @benpiper

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