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/encode.go

@@ -252,6 +252,17 @@ func encodeWordCheck(r backendclient.WordCheckResp) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeDraftView builds a DraftView payload wrapping the player's composition JSON. The
+// string is empty for the save acknowledgement (the client ignores that payload).
+func encodeDraftView(jsonStr string) []byte {
+	b := flatbuffers.NewBuilder(256)
+	j := b.CreateString(jsonStr)
+	fb.DraftViewStart(b)
+	fb.DraftViewAddJson(b, j)
+	b.Finish(fb.DraftViewEnd(b))
+	return b.FinishedBytes()
+}
+
 // encodeHistory builds a History payload (the decoded move journal).
 func encodeHistory(r backendclient.HistoryResp) []byte {
 	b := flatbuffers.NewBuilder(1024)