feat: add keyboard navigation and ARIA accessibility to customizer controls

[nishtha-agarwal-211]

Is your feature request related to a problem? Please describe.

The /customize page theme selector tiles and custom color controls currently rely heavily on mouse click interactions. Keyboard users and screen reader users (Tab, Enter, Space, Arrow keys) cannot smoothly select themes or receive live announcements when selecting theme presets.

Describe the solution you'd like

Enhance customizer components (app/customize/ and components/customizer/) with WCAG 2.1 AA compliant accessibility:

1. Add tabIndex={0}, role="radiogroup", and role="radio" attributes to theme tiles.
2. Add aria-checked and live region announcements (aria-live="polite") when a theme preset is applied.
3. Support arrow key navigation (ArrowLeft / ArrowRight) to cycle through available themes.
4. Ensure visible focus indicators (focus-visible:ring-2) on all interactive buttons and pickers.

Describe alternatives you've considered

• Relying on native browser defaults (does not style custom theme tiles or provide screen reader feedback on color changes).

[nishtha-agarwal-211]

/claim

[github-actions]

🎉 Assigned! Welcome to the project, @nishtha-agarwal-211.

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