fix: Prevent mobile homepage overflow (#13453)
## Summary
- prevent homepage grid cells from creating an implicit second mobile
column
- constrain feature illustrations to their containers at narrow
breakpoints
- keep the compute-saved label on one line with contained mobile sizing
and spacing
- keep existing desktop illustration and counter sizes
## Testing
- `pnpm --dir apps/docs collect-examples-data`
- `pnpm --dir apps/docs check-types`
- browser validation at 320px, 390px, 400px, 430px, 433px, 458px, 459px,
498px, and 1280px widths
- verified counter number and label bounds, spacing, and document width
- pre-push format and TOML checks
