🎨 Palette: Accessible Table Sorting & Base Layout Fix

[Woschj]

This PR introduces micro-UX and accessibility improvements to the Scandy interface.

♿ Accessibility Improvements

• Accessible Table Sorting: The dynamically generated table header wrappers now follow ARIA best practices. They are focusable, have the correct roles/labels, and respond to keyboard input. Assistive technologies will now also announce the current sort state thanks to the aria-sort attribute.
• Valid HTML Structure: Fixed a glaring syntax error in the core base.html template where a div was closed with a button tag. This ensures the accessibility tree is correctly constructed.

✨ Visual Polish

• Added Tailwind focus ring styles to sortable headers to provide clear visual feedback for keyboard users.

Verified with unit tests and code inspection.

---

PR created automatically by Jules for task 1802482005209508516 started by @Woschj

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.