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/mock/client.ts

@@ -93,6 +93,7 @@ export class MockGateway implements GatewayClient {
   private blocks: AccountRef[] = [];
   private invitations: Invitation[] = mockInvitations();
   private readonly stats: Stats = { ...MOCK_STATS };
+  private readonly drafts = new Map<string, string>();
 
   constructor() {
     // Seed the per-variant alphabet cache the rack, blank chooser and scoring read, so the
@@ -230,6 +231,7 @@ export class MockGateway implements GatewayClient {
     g.rack.push(...draw(variant, drawn));
     g.bagLen -= drawn;
     g.view.toMove = (seat + 1) % g.view.players;
+    this.drafts.delete(gameId);
     this.scheduleOpponentReply(gameId);
     return { move: structuredClone(move), game: structuredClone(g.view) };
   }
@@ -263,6 +265,7 @@ export class MockGateway implements GatewayClient {
     };
     g.moves.push(move);
     g.view.moveCount += 1;
+    this.drafts.delete(gameId);
     if (action === 'resign') {
       g.view.status = 'finished';
       g.view.endReason = 'resignation';
@@ -319,6 +322,15 @@ export class MockGateway implements GatewayClient {
   }
   async complaint(): Promise<void> {}
 
+  // --- draft (Stage 17): an in-memory composition store, so the reload/off-turn flow is
+  // exercised without a backend. A committed move clears the actor's own draft, as on the server.
+  async draftGet(gameId: string): Promise<string> {
+    return this.drafts.get(gameId) ?? '';
+  }
+  async draftSave(gameId: string, json: string): Promise<void> {
+    this.drafts.set(gameId, json);
+  }
+
   // --- chat ---
   async chatPost(gameId: string, body: string): Promise<ChatMessage> {
     const g = this.game(gameId);