guide-viewer: readable on phones and tablets, desktop untouched - #1329

fix(guide-viewer): readable on phones and tablets, desktop untouched

Every change is behind a breakpoint; 1440px and 1024px renders of the same
guide are byte-identical before and after (screenshot MD5s match).

• Split diffs below lg (1024px) are forced unified in the portable viewer's
  diff renderer (matchMedia; the setting is untouched, so a wider window
  gets split back). A phone has ~350px of pane and a portrait tablet ~430px,
  so two columns were under 220px each.
• Padding scales: page px-3/sm:px-6/lg:px-10, chapter column px-4/md:px-6,
  diff column px-1.5/md:px-4. Code pane on a 390px phone: 276px → 352px.
• Tablets: the chapter column is proportional (minmax(260px,36%)) from md
  and the fixed 440px only from lg. Pane at 768px: 214px → 426px.
• Header actions (Download, theme) sit in a right-aligned row above the
  title below md instead of floating into it.

Viewer rebuilt and published (viewer.dWt7KCum.js), manifest synced.

fix(guide-viewer): touch targets, labels, and no tap delay on coarse pointers

Only under pointer: coarse (Tailwind's pointer-coarse: variant), so mouse
layouts are unchanged:

• Reviewed checkbox and the collapse chevron get an invisible ::before hit
  area (visual 15–17px, hit ≥ 44px); the "Reviewed" text button and file
  chips get taller padding; the theme toggle and hosted Download button grow
  to a 44px hit box.
• touch-action: manipulation on controls in the portable viewer and the
  landing page (no double-tap-to-zoom delay; the page still pinch-zooms).
• aria-label on the two icon-only buttons (theme toggle, collapse chevron).
• Landing page: the GitHub link and the Copy button are 44px tall on touch.

Tailwind v4 already gates hover: behind @media (hover: hover), so no
false hover states on tap. Viewer rebuilt and published, manifest synced.