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/index.css

@@ -583,7 +583,7 @@ body {
     opacity: 0.9;
   }
 
-  /* ===== E10 S1c · the minimum-target half of the 1024x768 comfort contract =====
+  /* ===== E10 S1c · the minimum-target half of the tablet comfort contract (docs/DESIGN.md) =====
      A 32px icon button is fine with a mouse (a precise pointer plus hover to confirm the aim) and
      not fine with a thumb. So the minimum grows ONLY where there is no precise pointer, which on
      this app's real second device — a tablet — is exactly where it is needed.