Update invoice skeleton to use boolean for is_mixed and add export fields; enhance mergeDefaults function to filter undefined values

frontend/src/routes/dashboard/invoices/edit/[id]/+page.svelte

@@ -153,7 +153,7 @@
 	const othersSkeleton = {
 		comments_status: '',
 		transport_mode: 'TRUCK',
-		is_mixed: null,
+		is_mixed: false,
 		print_stamp: false,
 		rule_3121_parties_ii: false,
 		related_doc_id: null,
@@ -165,7 +165,15 @@
 		operation_num: '',
 		adendas: '',
 		observations_vu: '',
-		certified_number: ''
+		certified_number: '',
+		// Export fields
+		bill_number: '',
+		guide_number: '',
+		shipment_number: '',
+		option_iv18: '',
+		delivered_status: false,
+		received_by: '',
+		delivery_date: ''
 	};
 
 	const continuationSkeleton = {
@@ -196,7 +204,11 @@
 
 	function mergeDefaults(skeleton: any, partial: any) {
 		if (!partial) return skeleton;
-		return { ...skeleton, ...partial };
+		// Filter out undefined values from partial to preserve skeleton defaults
+		const filtered = Object.fromEntries(
+			Object.entries(partial).filter(([_, value]) => value !== undefined)
+		);
+		return { ...skeleton, ...filtered };
 	}
 
 	// Referencias a los componentes de formulario para obtener sus datos
@@ -503,8 +515,16 @@
 	}
 
 	let isLoadingDefaults = $state(false);
-	let lastLoadedKey = $state(
-		data.defaultSettings
+	
+	// Compute initial key from source data to avoid state reference warning
+	const initialLoadedKey = data.defaultSettings?.InvoiceTopFieldsFormData
+		? `${data.defaultSettings.InvoiceTopFieldsFormData.invoice_type || ''}-${data.defaultSettings.InvoiceTopFieldsFormData.operation_type || ''}`
+		: '';
+	let lastLoadedKey = $state(initialLoadedKey);
+
+	// Create a derived value for the current key to track changes reactively
+	let currentKey = $derived(
+		InvoiceTopFieldsFormData?.invoice_type && InvoiceTopFieldsFormData?.operation_type
 			? `${InvoiceTopFieldsFormData.invoice_type}-${InvoiceTopFieldsFormData.operation_type}`
 			: ''
 	);
@@ -518,7 +538,6 @@
 		)
 			return;
 
-		const currentKey = `${InvoiceTopFieldsFormData.invoice_type}-${InvoiceTopFieldsFormData.operation_type}`;
 		if (currentKey === lastLoadedKey) return;
 
 		isLoadingDefaults = true;