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/codec.ts

@@ -73,6 +73,18 @@ export function encodeStateRequest(gameId: string, includeAlphabet: boolean): Ui
   return finish(b, fb.StateRequest.endStateRequest(b));
 }
 
+// encodeDraftSave wraps the player's composition JSON (Stage 17). The string is opaque on the
+// wire — the gateway forwards it verbatim and only the client reads {rack_order, board_tiles}.
+export function encodeDraftSave(gameId: string, json: string): Uint8Array {
+  const b = new Builder(256);
+  const gid = b.createString(gameId);
+  const j = b.createString(json);
+  fb.DraftRequest.startDraftRequest(b);
+  fb.DraftRequest.addGameId(b, gid);
+  fb.DraftRequest.addJson(b, j);
+  return finish(b, fb.DraftRequest.endDraftRequest(b));
+}
+
 export function encodeSubmitPlay(
   gameId: string,
   dir: 'H' | 'V',
@@ -359,6 +371,13 @@ export function decodeWordCheck(buf: Uint8Array): WordCheckResult {
   return { word: s(r.word()), legal: r.legal() };
 }
 
+// decodeDraftView returns the player's stored composition JSON (empty when none is stored or
+// for the save acknowledgement); the caller parses {rack_order, board_tiles}.
+export function decodeDraftView(buf: Uint8Array): string {
+  const v = fb.DraftView.getRootAsDraftView(new ByteBuffer(buf));
+  return v.json() ?? '';
+}
+
 export function decodeHistory(buf: Uint8Array): History {
   const h = fb.History.getRootAsHistory(new ByteBuffer(buf));
   const moves: MoveRecord[] = [];