Stage 7 (wip): docs bake + stage renumber (insert UI Stage 8, shift +1)

- PLAN.md: new Stage 8 (UI social/account/history); Telegram->9, Admin->10, Linking->11, Polish->12; tracker + Stage 7 refinements; split the Stage 6 'wired in Stage 7' note between 7 and 8
- ARCHITECTURE: promote ui to current (slice scope, board-replay, codegen, theming, mock)
- FUNCTIONAL(+ru): client-app section with the Stage 7/8 split
- README + ui/README + CLAUDE.md: UI build/run/test, codegen, pnpm notes
- bumped Stage 8-11 refs (+1) across docs and code comments

CLAUDE.md

@@ -121,4 +121,12 @@ go vet ./backend/...
 gofmt -l .                      # must print nothing
 go test -count=1 ./backend/...
 go run ./backend/cmd/backend    # /healthz, /readyz on :8080
+
+cd ui && pnpm install && pnpm check && pnpm test:unit && pnpm build  # the UI (Stage 7+)
+pnpm start                      # UI mock mode: lobby -> game, no backend
 ```
+
+The `ui` module is a Node project (pnpm), **not** in `go.work`; its CI is
+`.gitea/workflows/ui-test.yaml`. Committed edge codegen under `ui/src/gen/`
+(regenerate with `pnpm codegen`); pnpm build-script approval lives in
+`ui/pnpm-workspace.yaml` (`allowBuilds: esbuild: true`).