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

backend/internal/server/handlers_admin.go

@@ -9,7 +9,7 @@ import (
 // The /api/v1/admin/* endpoints are reached through the gateway's Basic-Auth
 // reverse proxy (docs/ARCHITECTURE.md §12). The backend trusts the gateway to
 // have authenticated the operator; the admin surface itself (complaint review,
-// dictionary versions) lands in Stage 9. handleAdminPing is the proxy target that
+// dictionary versions) lands in Stage 10. handleAdminPing is the proxy target that
 // proves the path end to end until then.
 func (s *Server) handleAdminPing(c *gin.Context) {
 	c.JSON(http.StatusOK, gin.H{"status": "ok"})