fix: prevent mobile setup grid overflow

[aryamthecodebreaker]

Summary

• allow both setup grid columns and their contents to shrink below min-content width
• preserve the responsive one-column layout with minmax(0, 1fr)`n- prevent the Get Started Action section from widening a 390px viewport to 427px

Verification

• web lint passed
• production web build passed
• real Playwright at 390x844: clientWidth 390, scrollWidth 390
• browser console: 0 errors, 0 warnings

Closes #442