feat: show expected withholding tax on Sales Invoice

Custom fields Tax Withholding Category (fetched from Customer),
Withholding Tax and Net Payable After Withholding Tax on Sales Invoice;
computed in the Thai set_tax_withholding override from the category
rate and single threshold, informational only (totals/GL unchanged),
printed under Rounded Total by the standard layout. Payment Entry now
prefers each invoice's category over the customer's.

default_thai_company/hooks.py

@@ -102,6 +102,20 @@ fixtures = [
 		],
 	},
 	{"doctype": "Tax Withholding Category", "filters": [["name", "like", "WHT %"]]},
+	{
+		"doctype": "Custom Field",
+		"filters": [
+			[
+				"name",
+				"in",
+				[
+					"Sales Invoice-tax_withholding_category",
+					"Sales Invoice-withholding_tax_amount",
+					"Sales Invoice-amount_after_withholding",
+				],
+			]
+		],
+	},
 ]
 
 # Installation