Stage 7 polish: docs + mark Stage 7 done (Part H)

- new docs/UI_DESIGN.md (design system: shell, nav, tab-bar, tiles, board zoom/labels, HoldConfirm, banner rotator, result iconography)
- ARCHITECTURE: app-shell + announcement channel (mock->server) + hint place-on-board / no_hint_available contract + board-style setting
- FUNCTIONAL(+ru): hint-on-board, word-check constraints/throttle, board style
- PLAN: Stage 7 -> done + polish refinement note; CLAUDE sources list

CLAUDE.md

@@ -13,6 +13,7 @@ conversation memory — is the source of continuity. Keep it that way.
 - [`docs/FUNCTIONAL.md`](docs/FUNCTIONAL.md) (+ [`_ru`](docs/FUNCTIONAL_ru.md)
   mirror) — per-domain user stories. English authoritative.
 - [`docs/TESTING.md`](docs/TESTING.md) — test layers + the per-stage CI gate.
+- [`docs/UI_DESIGN.md`](docs/UI_DESIGN.md) — the `ui` visual/interaction design system.
 
 ## Mandatory per-stage workflow