feat(v2): make audio history player keyboard-accessible

[RahulHarihar]

Summary

Fixes #184

In desktop/src/components/AudioPlayer.tsx, the waveform seeker declared role="slider" with ARIA min/max/now attributes, but lacked focusability and keyboard event handlers.

This PR completes the W3C APG Slider keyboard contract for the audio player:

• Focusability: Added tabIndex={peaks ? 0 : -1} with a subtle focus indicator (focus-visible:ring-2 focus-visible:ring-accent/50), consistent with the existing disabled={!peaks} guard on the Play button.
• Keyboard seeking: Added onKeyDown handler:
  • ← / →: seek backward / forward by 5 seconds
  • Home / End: jump to 0:00 / track end
  • Space / Enter: toggle play / pause
• Screen reader readability: Added aria-valuetext announcing formatted time (e.g., 0:15 of 1:20) via formatClock.
• Zero regression: Mouse click seeking and waveform visual rendering are untouched. Confined to one file with no new dependencies.

Tested On

• Windows 11:
  • npm run typecheck passed cleanly
  • npm test -- --run passed cleanly
  • Verified keyboard focus ring and arrow/Home/End/Space seeking in browser preview