battle-fetch: migrate to user.games.battle ConnectRPC command

The Phase 27 BattleViewer was the last UI surface still issuing raw
fetch() against the backend REST contract (`/api/v1/user/games/...
/battles/...`). The dev-deploy gateway never proxied that path, so
the viewer worked only in tools/local-dev/. Move it onto the signed
ConnectRPC channel every other authenticated surface already uses.

Wire pieces:
- FBS GameBattleRequest in pkg/schema/fbs/battle.fbs, regenerated
  Go + TS bindings.
- MessageTypeUserGamesBattle constant + GameBattleRequest struct in
  pkg/model/report/messages.go.
- pkg/transcoder/battle.go gains GameBattleRequestToPayload and
  PayloadToGameBattleRequest helpers.
- gateway games_commands.go switches on the new message type and
  GETs /api/v1/user/games/{id}/battles/{turn}/{battle_id}; the JSON
  response is re-encoded as a FlatBuffers BattleReport before being
  returned. 404 from backend surfaces as the canonical `not_found`
  gateway error.
- ui/frontend/src/api/battle-fetch.ts now builds the FBS request,
  calls GalaxyClient.executeCommand, and decodes the FBS response
  into the existing UI shape (Record<string,string> race/ship maps,
  string-form UUID). BattleFetchError carries an HTTP-style status
  derived from the result code so the active-view's not_found branch
  keeps working.
- battle.svelte pulls the GalaxyClient from the in-game shell
  context. While the layout's boot Promise.all is in flight the
  effect stays in `loading` until the client handle becomes
  non-null.
- ui/Makefile FBS_INPUTS gains battle.fbs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pkg/transcoder/battle.go

@@ -382,3 +382,55 @@ func encodeBattleOffsetVector(
 	}
 	return builder.EndVector(length)
 }
+
+// GameBattleRequestToPayload converts model.GameBattleRequest to
+// FlatBuffers bytes suitable for the authenticated gateway transport.
+func GameBattleRequestToPayload(req *model.GameBattleRequest) ([]byte, error) {
+	if req == nil {
+		return nil, errors.New("encode game battle request payload: request is nil")
+	}
+
+	builder := flatbuffers.NewBuilder(64)
+
+	gameHi, gameLo := uuidToHiLo(req.GameID)
+	battleHi, battleLo := uuidToHiLo(req.BattleID)
+
+	fbs.GameBattleRequestStart(builder)
+	fbs.GameBattleRequestAddGameId(builder, fbs.CreateUUID(builder, gameHi, gameLo))
+	fbs.GameBattleRequestAddTurn(builder, uint32(req.Turn))
+	fbs.GameBattleRequestAddBattleId(builder, fbs.CreateUUID(builder, battleHi, battleLo))
+	offset := fbs.GameBattleRequestEnd(builder)
+	fbs.FinishGameBattleRequestBuffer(builder, offset)
+
+	return builder.FinishedBytes(), nil
+}
+
+// PayloadToGameBattleRequest converts FlatBuffers payload bytes into
+// model.GameBattleRequest.
+func PayloadToGameBattleRequest(data []byte) (result *model.GameBattleRequest, err error) {
+	if len(data) == 0 {
+		return nil, errors.New("decode game battle request payload: data is empty")
+	}
+
+	defer func() {
+		if recovered := recover(); recovered != nil {
+			result = nil
+			err = fmt.Errorf("decode game battle request payload: panic recovered: %v", recovered)
+		}
+	}()
+
+	req := fbs.GetRootAsGameBattleRequest(data, 0)
+	gameID := req.GameId(nil)
+	if gameID == nil {
+		return nil, errors.New("decode game battle request payload: game_id is missing")
+	}
+	battleID := req.BattleId(nil)
+	if battleID == nil {
+		return nil, errors.New("decode game battle request payload: battle_id is missing")
+	}
+	return &model.GameBattleRequest{
+		GameID:   uuidFromHiLo(gameID.Hi(), gameID.Lo()),
+		Turn:     uint(req.Turn()),
+		BattleID: uuidFromHiLo(battleID.Hi(), battleID.Lo()),
+	}, nil
+}