Improve mobile/touch support and accessibility

[TheSevenPens]

Summary

From FUTURES.md:

• pen-mouse.js is keyboard + mouse only; touch can view but not reposition the pen
• Control panel is fixed ~250px and not responsive
• Minimal ARIA on sliders; camera UI lacks dialog semantics
• No :focus-visible styles; WebGL canvas has no text alternative
• Some contrast may fail WCAG

Suggested scope (can split later)

1. Touch-friendly pen positioning (or documented limitation)
2. Responsive layout for small screens
3. Slider ARIA + focus styles
4. Short accessible description for the viewer

Acceptance

• Keyboard users can see focus
• Basic a11y attributes on primary controls
• Documented touch story (supported or explicitly not)