fix: accessibility for apperance

[kristian-zendato]

Summary

• Give theme preview images meaningful alternative text instead of empty alt.
• Present background options as a single HTML list while keeping the layout (custom / plain color / default on the first row, shipped backgrounds on the next).
• Make each theme option a focusable control that activates the theme on click/keyboard, and keep the visual radio/checkbox out of the tab order (tabindex="-1").

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[susnux]

This is not valid you cannot have a checkbox inside a button.
This should be just a DIV element and checkbox within already is the semantic input element.

[susnux]

So the outer elements needs to be a div and the outer fieldset needs a role="radiogroup".

[susnux]

why?
This is semantically not really correct, fieldset already was correct.
As this is a group of related input fileds. Its not really a list.

[kristian-zendato]

https://github.com/nextcloud-gmbh/customer-feature-requests/issues/1624

Related to this feedback, they want the background to be HTML list.

[susnux]

But thats invalid see my feedback on the issue. IMHO a false-positive text and should be closed as wont fix with proper reference to WCAG rules as the current semantics are correct (expect my other comment above).

[susnux]

See my comment - parially the issue is wrong as semantics for WCAG needs radiogroup and radio within the theme selection.

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