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.dom.test.tsx

@@ -127,7 +127,7 @@ describe("Button primitive (S5)", () => {
   });
 
   it("carries the coarse-pointer minimum target, except on a link", () => {
-    // The 1024x768 comfort contract: the minimum lives in the primitive, so no call site can opt
+    // The tablet comfort contract: the minimum lives in the primitive, so no call site can opt
     // out of it by accident. A link is text in a sentence and is excluded by design.
     const { unmount } = render(<Button>Tap</Button>);
     expect(screen.getByRole("button", { name: "Tap" }).className).toContain("touch-target");