🎨 Palette: [UX improvement] Add fieldset to background removal radio group

[benpiper]

What: Replaced the generic

and grouping for the "Background Removal Method" radio buttons with a and in web_app/templates/index.html.
Why: Grouping radio buttons with a generic

and causes screen readers to announce the radio buttons without the overarching group context. Using a provides this semantic context.
Before/After: The visual design remains identical thanks to inline CSS resetting the default borders and margins, but the underlying DOM is structurally sound.
Accessibility: Screen reader users will now correctly hear "Background Removal Method" announced as the group name when navigating to the radio buttons, improving overall form accessibility.

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PR created automatically by Jules for task 3755162760283523161 started by @benpiper

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