feat(reports): add Excel export to balance monitoring endpoint

Add ?export=excel support to GetBalanceMonitoring. Creates a new
repport.balance_monitoring.export.go with a 2-row merged header layout
matching the UI (Penjualan Ayam and Penjualan Telur grouped columns),
a totals row, red styling for negative Saldo Akhir, and frozen panes
below the header rows. Exported data reflects all active query filters.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/modules/repports/controllers/repport.controller.go

@@ -555,6 +555,10 @@ func (c *RepportController) GetBalanceMonitoring(ctx *fiber.Ctx) error {
 		return err
 	}
 
+	if isBalanceMonitoringExcelExportRequest(ctx) {
+		return exportBalanceMonitoringExcel(ctx, result, totals)
+	}
+
 	limit := query.Limit
 	if limit < 1 {
 		limit = 10