fix(Carousel): make slide indicators keyboard accessible

[ifeichuan]

Summary

Fixes #976. The Carousel slide indicators were rendered as <motion.div> elements with only an onClick handler, so keyboard-only users could neither focus them with Tab nor activate them with Enter/Space.

This switches the indicators to native <motion.button type="button">, which is focusable and Enter/Space-activatable for free, and adds the supporting a11y semantics:

• aria-label="Go to slide N" — screen readers now announce each control (previously empty <div>)
• aria-current — exposes which slide is active
• button resets (border/padding/appearance in CSS variants, border-0 p-0 appearance-none in Tailwind variants) — keep the dot styling identical
• a :focus-visible outline — keyboard focus is now visible (no outline on mouse click)

Mouse, drag and touch behaviour are unchanged.

Updated across all 4 variants (JS/TS × CSS/Tailwind), per the contributing guidelines.

How it was tested

Ran the demo locally (/components/carousel) and verified in a real browser:

Check	Result
Indicators are <button> with type="button", aria-label, aria-current	✅
Tab focuses each indicator in order (slide 1 → 2 → 3 → 4)	✅
Enter on a focused indicator changes the slide	✅
Space on a focused indicator changes the slide	✅
:focus-visible shows a 2px outline on keyboard focus, none on mouse click	✅

The focus-visible outline style follows the existing convention in the repo (e.g. LogoLoop, GlassSurface).

Note: registry artifacts under public/r/ are not included, matching how prior component PRs (e.g. #977) leave registry regeneration to the maintainer build.