docs(website): add wallpaper mode section and fix settings tab overflow

- Add Desktop Wallpaper Mode feature section with 4 cards covering
  WorkerW layer behaviour, Win+D survival, separate-process isolation,
  and Save-only settings apply
- Reduce settings tab padding (14px → 9px) so all five tabs fit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>