feat(Dock): add keyboard activation and accessible labels

[Sarthak-Bhagat2006]

Fixes #985 . The Dock component had role="button" and tabIndex={0} on items, making them focusable, but pressing Enter or Space did not trigger the onClick handler — unlike native <button> elements. Items also had no accessible label.

This adds keyboard activation and accessibility semantics:

• Added onKeyDown handler for Enter and Space to trigger onClick, with preventDefault() on Space to avoid page scroll.
• Added aria-label to each DockItem, derived from item.label when it's a plain string.

Mouse hover, magnification, and existing click behaviour are unchanged.

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

Note on aria-label

When label is a plain string, it's used as the aria-label. When label is JSX, the item falls back to existing behaviour (no aria-label) — a full fix for JSX labels would need a separate string prop, which felt out of scope here. Happy to add that if preferred.

How it was tested

Check	Result
Tab focuses each dock item	✅
Enter activates onClick	✅
Space activates onClick (no page scroll)	✅
Mouse hover/magnification still works	✅
Screen reader announces item label (string labels)	✅

Accessibility

• Keyboard-only users can now activate dock items
• Screen readers announce items with string labels via aria-label
• Follows WAI-ARIA Authoring Practices for the button role