feat(design): design lab + DESIGN.md skeleton (E28 S2a)

- docs/DESIGN.md: the written source of truth for the UI. Decided chapters
  (principles, device adaptation, action placement, text stays selectable,
  interaction rules, copy, anti-patterns, decision log); controls and
  surfaces are marked Pending until the owner picks from the lab.
- /design-lab (dev only): renders each open decision's variants with the
  app's real CSS, themes and font scale. Batch A = six control decisions.
- devOnlyGuard: a production build fails if anything under src/design-lab
  reaches the bundle (seen red with an unguarded import).
- Size contract: tablet portrait + landscape and up, replacing the
  1024x768 floor with its never-built "too small" message.

frontend/src/components/ui/README.md

@@ -46,10 +46,10 @@ domain-free.
 
 ## The size contract (E10 S1c)
 
-The app optimises for **1024x768 (tablet landscape) as the floor**; below it we owe an honest
-message, above it correct rendering all the way up. That contract lives in these components, not in
-the ~160 call sites, because a rule repeated in 160 places is a rule that is already broken
-somewhere:
+The app supports **tablet portrait and landscape and every size above them** (from 768px wide; phones
+are not a target — see [`docs/DESIGN.md`](../../../../docs/DESIGN.md) § Device adaptation). Measure a UI
+change at 1024x768, 820x1180 and a desktop size. That contract lives in these components, not in the
+~160 call sites, because a rule repeated in 160 places is a rule that is already broken somewhere:
 
 - **Geometry is a ladder, not a free choice.** `Button`'s `SIZES` are the rungs, derived from what
   the app actually used. Pick a rung; do not hand-roll padding on a primitive.