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/Button.tsx

@@ -155,7 +155,7 @@ const ButtonImpl = forwardRef<HTMLButtonElement, ButtonOwnProps>(function Button
   // here rather than by asking every call site to pass `size="none"` alongside `variant="link"` —
   // a pairing everyone would eventually forget, which is how a "link" grows a button's box.
   const resolvedSize: ButtonSize = size ?? (variant === "link" ? "none" : "md");
-  // The 1024x768 comfort contract lives HERE, not in the call sites: on a coarse pointer every
+  // The tablet comfort contract (docs/DESIGN.md) lives HERE, not in the call sites: on a coarse pointer every
   // button-shaped control gets at least a 2.75rem box (see .touch-target in index.css). `link` is
   // excluded on purpose — it is a run of text inside a sentence, and a 44px-tall inline link would
   // wreck the paragraph it sits in rather than make anything easier to hit.