chore: mobile updates ui

[ausbernard]

Mobile UI fixes — index + writing pages

• Horizontal overflow (index.html): Added overflow-x: hidden to body and max-width: 100% to .hero-body to prevent the 54ch
  constraint from pushing the page wider than the viewport on ~375px screens
• Contact button (both pages): Was hidden with display: none on mobile — now visible with tighter sizing (0.72rem / compact
  padding), nav gap tightened to fit everything on one line
• Open Source logos (index.html): .oss-repo was display: none on mobile — restored with updated grid (40px 1fr auto) and
  36px image sizing
• Writing cards (index.html): Reduced inner padding from 2rem to 1.25rem on mobile so full-width cards don't feel too
  sprawling
• Writing list (writing/index.html): Post row padding was 1.5rem 0 (no horizontal padding), causing content to sit flush
  against card edges — restored to 1.25rem 1rem. Tag filter gap tightened so all tags fit on one row. hero-sub gets max-width:
  100% override

Test

Verified on iPhone (375px portrait) in Safari.