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.

docs/development.md

@@ -32,6 +32,10 @@ For a fast frontend edit loop, run the Vite dev server with HMR against that API
 cd frontend && npm install && npm run dev   # http://localhost:5173, proxying the API on :8080
 ```
 
+The dev server also serves the **design lab** at `http://localhost:5173/design-lab`: the open design
+decisions and the chosen patterns, rendered with the app's real CSS. It never reaches a production build
+(see [`DESIGN.md`](DESIGN.md)).
+
 ## The gate
 
 A change is done when all of this is green. CI runs the same checks server-side