Stage 17 round 6 (#4/#5/#6): draft persistence wire + gateway + UI

Complete the client-side draft feature on top of the shipped backend
foundation (the game_drafts store/service):

- FB: DraftRequest{game_id,json} + DraftView{json} (a draft get reuses
  GameActionRequest); regenerated committed Go + TS bindings.
- Backend REST: GET/PUT /games/:id/draft, a draftDTO
  (rack_order/board_tiles) mapped to game.Draft.
- Gateway: draft.get/draft.save transcode forwarding the composition
  JSON verbatim (json.RawMessage both ways -- no double-encode).
- UI: debounced save of the rack order + board tiles and restore on
  load (lib/draft.ts), plus #5 -- tiles may be arranged on the
  opponent's turn (placement relaxed; the preview and Make-move stay
  your-turn-only, so an off-turn draft is position-only).

Tests: backend handler validation, gateway pass-through round-trip, UI
draft/codec units, and a draft-restore e2e.

backend/internal/server/handlers.go

@@ -66,6 +66,8 @@ func (s *Server) registerRoutes() {
 		u.POST("/games/:id/complaint", s.handleComplaint)
 		u.GET("/games/:id/history", s.handleHistory)
 		u.GET("/games/:id/gcg", s.handleExportGCG)
+		u.GET("/games/:id/draft", s.handleGetDraft)
+		u.PUT("/games/:id/draft", s.handleSaveDraft)
 	}
 	if s.matchmaker != nil {
 		u.POST("/lobby/enqueue", s.handleEnqueue)