feat(design): batch C recorded, the player row open as C5 (E28 S2d)

C1-C4 as proposed: the action classification, overlay-drawer side panels,
touch-first players, dialog focus by purpose with a pinned footer. The
players were then played live at tablet size, which found the Plex
control row running off-screen below 1024; how it fits is open as C5.
CLAUDE.md now points at DESIGN.md.

CLAUDE.md

@@ -46,7 +46,9 @@ how it gets built. Concretely:
   behind each option and the weight of the decision. These decision points are where the product owner
   engages with the build and learns from it, so make them understandable — a reasoned menu, not a bare one.
 - **Mockups for design/UX decisions.** For any visual or interaction choice, show mockup options to
-  choose from before building.
+  choose from before building. How the UI looks and behaves is decided in
+  [`docs/DESIGN.md`](docs/DESIGN.md) (its variants render in the dev-only `/design-lab`): build to it,
+  and change it only through its decision log.
 - **Challenge unsound requests.** If a request is technically ill-advised and you can back it with real
   arguments, push back or propose an alternative — don't comply blindly.
 - **Verify UI in the browser before calling it done.** tsc/unit/DOM tests miss visual and interaction