1.0.1

Onboarding stepper: fix alignment across all step states

The <button>-as-flex-column layout was collapsing inconsistently (done
steps dropped a row below the active/upcoming ones) and the progress line
extended past the first icon. Rebuilt it:
- each step button is a fixed-width slot; a .si span does the
  icon-over-label stacking, so button flex quirks can't offset it
- the track/fill run exactly between the first and last icon centres
  (--pad = side padding + half a step), and the JS progress fill uses the
  same --pad so it always lines up with the icons
- consistent vertical alignment for visited / active / upcoming

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>