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>

gateway/internal/backendclient/games_commands.go

@@ -63,6 +63,12 @@ func (c *RESTClient) ExecuteGameCommand(ctx context.Context, command downstream.
 			return downstream.UnaryResult{}, fmt.Errorf("backendclient: execute game command %q: %w", command.MessageType, err)
 		}
 		return c.executeUserGamesReport(ctx, command.UserID, req)
+	case reportmodel.MessageTypeUserGamesBattle:
+		req, err := transcoder.PayloadToGameBattleRequest(command.PayloadBytes)
+		if err != nil {
+			return downstream.UnaryResult{}, fmt.Errorf("backendclient: execute game command %q: %w", command.MessageType, err)
+		}
+		return c.executeUserGamesBattle(ctx, command.UserID, req)
 	default:
 		return downstream.UnaryResult{}, fmt.Errorf("backendclient: execute game command: unsupported message type %q", command.MessageType)
 	}
@@ -127,6 +133,26 @@ func (c *RESTClient) executeUserGamesReport(ctx context.Context, userID string,
 	return projectUserGamesReportResponse(status, respBody)
 }
 
+func (c *RESTClient) executeUserGamesBattle(ctx context.Context, userID string, req *reportmodel.GameBattleRequest) (downstream.UnaryResult, error) {
+	if req.GameID == uuid.Nil {
+		return downstream.UnaryResult{}, errors.New("execute user.games.battle: game_id must not be empty")
+	}
+	if req.BattleID == uuid.Nil {
+		return downstream.UnaryResult{}, errors.New("execute user.games.battle: battle_id must not be empty")
+	}
+	target := fmt.Sprintf("%s/api/v1/user/games/%s/battles/%d/%s",
+		c.baseURL,
+		url.PathEscape(req.GameID.String()),
+		req.Turn,
+		url.PathEscape(req.BattleID.String()),
+	)
+	respBody, status, err := c.do(ctx, http.MethodGet, target, userID, nil)
+	if err != nil {
+		return downstream.UnaryResult{}, fmt.Errorf("execute user.games.battle: %w", err)
+	}
+	return projectUserGamesBattleResponse(status, respBody)
+}
+
 // buildEngineCommandBody serialises a slice of typed commands into the
 // JSON shape expected by backend's command/order handlers (a
 // `gamerest.Command` with the actor field left empty — backend rebinds
@@ -262,3 +288,32 @@ func projectUserGamesReportResponse(statusCode int, payload []byte) (downstream.
 		return downstream.UnaryResult{}, fmt.Errorf("unexpected HTTP status %d", statusCode)
 	}
 }
+
+// projectUserGamesBattleResponse decodes the engine's BattleReport JSON
+// payload (forwarded by backend's user.games.battle proxy) and
+// re-encodes it as a FlatBuffers BattleReport for the signed-gRPC
+// client. 404 from backend surfaces as the canonical `not_found`
+// gateway error so the UI can render its "battle not found" state.
+func projectUserGamesBattleResponse(statusCode int, payload []byte) (downstream.UnaryResult, error) {
+	switch {
+	case statusCode == http.StatusOK:
+		var report reportmodel.BattleReport
+		if err := json.Unmarshal(payload, &report); err != nil {
+			return downstream.UnaryResult{}, fmt.Errorf("decode engine battle report: %w", err)
+		}
+		encoded, err := transcoder.BattleReportToPayload(&report)
+		if err != nil {
+			return downstream.UnaryResult{}, fmt.Errorf("encode battle report payload: %w", err)
+		}
+		return downstream.UnaryResult{
+			ResultCode:   userCommandResultCodeOK,
+			PayloadBytes: encoded,
+		}, nil
+	case statusCode == http.StatusServiceUnavailable:
+		return downstream.UnaryResult{}, downstream.ErrDownstreamUnavailable
+	case statusCode >= 400 && statusCode <= 599:
+		return projectUserBackendError(statusCode, payload)
+	default:
+		return downstream.UnaryResult{}, fmt.Errorf("unexpected HTTP status %d", statusCode)
+	}
+}