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

docs/ARCHITECTURE.md

@@ -38,6 +38,11 @@ Three executables plus per-platform side-services:
   is a hash router and the session token is held in memory + IndexedDB. A build-flagged
   in-memory mock transport (`pnpm start`) runs the whole slice with no backend.
   Embeddable in platform webviews; packageable to native (iOS/Android) via Capacitor.
+  The client uses a mobile-app shell (a growing nav bar; content pinned to the bottom),
+  a one-line **announcement banner** under the nav (a client-side mock rotation today —
+  a server-driven channel later, §10), and a client **board-style** setting (bonus-label
+  mode). The visual/interaction design system is documented in
+  [`UI_DESIGN.md`](UI_DESIGN.md).
 - **`platform/<name>`** *(planned)* — per-platform side-services (Telegram bot
   first): deep-link invites and platform-native push notifications. They talk
   to `backend` over an internal API.
@@ -209,7 +214,11 @@ Key points:
   (`hint_balance`, spent after the allowance; top-ups are a later feature). A hint
   reveals the top-1 ranked move (`GenerateMoves[0]`). The lobby/tournament caller
   picks the per-game defaults (e.g. one in casual random games, none in
-  tournaments).
+  tournaments). The client **lays the hinted tiles onto the board** as a pending
+  placement and leaves the commit to the player. When the rack has no legal move the
+  service spends **nothing** and returns `ErrNoHintAvailable` — surfaced as the distinct
+  result code `no_hint_available` (separate from `hint_unavailable`) so the UI can say
+  "no options" rather than "no hints left".
 - **Word-check tool**: unlimited dictionary lookups against the game's pinned
   dictionary; each result offers a **complaint** (complainant, game, variant,
   dict_version, word, the disputed result, an optional note) that lands in an
@@ -363,6 +372,10 @@ match-found. Out-of-app platform push (your-turn, nudge) is wired in Stage 9;
 session-revocation events and cursor-based stream resume are deferred
 (single-instance MVP).
 
+A separate **announcements channel** feeds the client's one-line banner (UI_DESIGN.md).
+It is a client-side **mock** rotation today; a server-driven source (operational notices,
+promotions) is future work and would deliver short markdown messages (text + links).
+
 ## 11. Observability
 
 - Structured logging with `go.uber.org/zap` (JSON). OpenTelemetry tracer and