Improve demo gallery mobile view #69

Problem

The demo gallery drawer currently uses a fixed 480px width and doesn't have mobile-specific optimizations. On smaller screens the layout may feel cramped or awkward.

Suggested improvements

• Full-screen drawer on mobile (< 640px) instead of side panel
• Touch-friendly tap targets for demo cards and category filter chips
• Horizontal scroll for category filter with momentum scrolling
• Bottom sheet style on mobile instead of right-side drawer
• Ensure the "Demos" header button is accessible on narrow viewports

Context

Introduced in #68.