Add cursor gravity toggle button

Details

Add a third floating toggle, stacked above the weather button, that turns
the mesh background's cursor "gravity well" interaction on and off.
- New #gravity-toggle-btn in the v2 layout with an orbit/gravity-well icon
  (a central mass with an elliptical orbit) matching the existing glass
  toggle styling and off-state slash.
- mesh-background.ts gates the pointer warp behind a meshGravityEnabled
  flag persisted in localStorage('gravity'); the grid still always draws.
  Disabling releases the well so the sheet eases back flat.
- Default is on for fine pointers (mouse/trackpad) and off for touch
  devices (pointer: coarse), since the effect needs a hovering cursor.
- SCSS: desktop bottom 136px, mobile 124px, plus the backdrop-filter
  fallback and extra footer clearance for the taller dock.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NyxwQRZRNgsrhdBYVQvqNg

Files changed (4)