feat(CircularGallery): add keyboard navigation support

[Sarthak-Bhagat2006]

Fixes #981. The CircularGallery component could only be navigated using mouse drag, touch gestures, and mouse wheel scrolling. Keyboard-only users could not interact with the gallery because it was not focusable and did not respond to keyboard input.

This adds keyboard navigation support and accessibility semantics:

• Added ArrowLeft and ArrowRight keyboard navigation.
• Added tabIndex={0} so the gallery can receive keyboard focus.
• Added role="region" and a descriptive aria-label for assistive technologies.
• Added keyboard event listener cleanup on component unmount.

Mouse, touch, drag, and wheel behaviour are unchanged.

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

How it was tested

Ran the demo locally and verified in a real browser:

Check	Result
Gallery receives focus via Tab	✅
ArrowLeft navigates to previous item	✅
ArrowRight navigates to next item	✅
Mouse drag interaction still works	✅
Mouse wheel interaction still works	✅
Touch interaction still works	✅
Keyboard event listeners are cleaned up on unmount	✅

Accessibility

• Keyboard-only users can now navigate the gallery.
• The component is focusable through normal tab navigation.
• Screen readers announce the gallery region using the provided aria-label.