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.

ui/src/lib/client.ts

@@ -83,6 +83,13 @@ export interface GatewayClient {
   checkWord(gameId: string, word: string, variant: Variant): Promise<WordCheckResult>;
   complaint(gameId: string, word: string, note: string): Promise<void>;
 
+  // --- draft (Stage 17) ---
+  /** The player's server-persisted client-side composition (rack order + board tiles), so a
+   *  reload or a second device resumes the same arrangement. The JSON is opaque to the
+   *  gateway; the client owns the {rack_order, board_tiles} shape. */
+  draftGet(gameId: string): Promise<string>;
+  draftSave(gameId: string, json: string): Promise<void>;
+
   // --- chat ---
   chatPost(gameId: string, body: string): Promise<ChatMessage>;
   chatList(gameId: string): Promise<ChatMessage[]>;