Enhance invoice processing and validation logic

- Updated `InvoiceService` to accurately reflect the number of line items in `party_count` for invoices.
- Modified weight calculations in the create and update validators to ensure consistent handling of weight types, now using lowercase comparison for "KGS".
- Adjusted invoice data structure in the frontend to include `total_items` and refined logistics handling.
- Improved table rendering in the invoice edit component for better sticky header behavior and item visibility.

These changes aim to improve data integrity and user experience in invoice management.

frontend/src/lib/api/dashboard/a76/invoices.ts

@@ -232,13 +232,14 @@ export interface Invoice {
 	download_substance?: boolean | null;
 	download_class?: boolean | null;
 	download_def?: boolean | null;
+	total_items?: number | null;
 	payment_terms?: string | null;
 	handling_fees?: number | null;
 	option_iv18?: string | null;
 	enajenation_goods?: boolean | null;
 	compliance_mx?: InvoiceComplianceMx | null;
 	financials?: InvoiceFinancials | null;
-	logistics?: InvoiceLogistics[];
+	logistics?: InvoiceLogistics | null;
 	details?: InvoiceSalesDetails[];
 	collections?: InvoiceCollections[];
 	// Client-side only properties