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/eslint.config.js

@@ -357,6 +357,13 @@ export default tseslint.config(
     files: ["src/components/ui/FocusSafeButton.tsx", "src/components/ui/AnchoredPopover.tsx"],
     rules: { "local/focus-safe-disabled": "off" },
   },
+  {
+    // The design lab (src/design-lab, E28) is a dev-only page that never reaches the production
+    // bundle (devOnlyGuard fails the build if it does). Its copy is the developer's and the product
+    // owner's working text, not the product's, so the translate-every-label rule does not apply.
+    files: ["src/design-lab/**/*.{ts,tsx}"],
+    rules: { "local/literal-a11y-text": "off" },
+  },
   {
     // Tests stand doubles in for the network (`vi.stubGlobal("fetch", …)`) and may reach it directly.
     // …and render throwaway markup whose labels no user ever hears.