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.

gateway/internal/transcode/transcode.go

@@ -7,6 +7,7 @@ package transcode
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 
 	"scrabble/gateway/internal/backendclient"
@@ -38,6 +39,8 @@ const (
 	MsgGameHistory    = "game.history"
 	MsgChatList       = "chat.list"
 	MsgChatNudge      = "chat.nudge"
+	MsgDraftGet       = "draft.get"
+	MsgDraftSave      = "draft.save"
 )
 
 // Request is one decoded Execute call.
@@ -108,6 +111,8 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLan
 	r.ops[MsgGameHistory] = Op{Handler: historyHandler(backend), Auth: true}
 	r.ops[MsgChatList] = Op{Handler: chatListHandler(backend), Auth: true}
 	r.ops[MsgChatNudge] = Op{Handler: nudgeHandler(backend), Auth: true}
+	r.ops[MsgDraftGet] = Op{Handler: getDraftHandler(backend), Auth: true}
+	r.ops[MsgDraftSave] = Op{Handler: saveDraftHandler(backend), Auth: true}
 	registerStage8(r, backend)
 	registerStage11(r, backend, tg, defaultLanguages)
 	return r
@@ -421,3 +426,28 @@ func nudgeHandler(backend *backendclient.Client) Handler {
 		return encodeChat(res), nil
 	}
 }
+
+// getDraftHandler returns the player's saved composition (Stage 17). It reuses
+// GameActionRequest for the game id and wraps the backend's raw JSON in a DraftView.
+func getDraftHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsGameActionRequest(req.Payload, 0)
+		raw, err := backend.GetDraft(ctx, req.UserID, string(in.GameId()))
+		if err != nil {
+			return nil, err
+		}
+		return encodeDraftView(string(raw)), nil
+	}
+}
+
+// saveDraftHandler upserts the player's composition (Stage 17), forwarding the opaque JSON
+// string verbatim. It echoes an empty DraftView as a well-formed acknowledgement.
+func saveDraftHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		in := fb.GetRootAsDraftRequest(req.Payload, 0)
+		if err := backend.SaveDraft(ctx, req.UserID, string(in.GameId()), json.RawMessage(in.Json())); err != nil {
+			return nil, err
+		}
+		return encodeDraftView(""), nil
+	}
+}