feat: customer-side withholding tax on Payment Entry

Thai customers withhold tax at payment; ERPNext's customer-side handling
is Indian TCS which adds tax on top of the Sales Invoice. Override
SalesInvoice.set_tax_withholding to no-op for Thai companies, expose
"Apply Tax Withholding Amount" on Customer/Receive Payment Entries, and
on validate compute rate x pre-VAT amount of each allocated reference
(prorated, single threshold honoured) as a deduction to "Withholding Tax
Receivable". Client script fetches the Customer's category when the box
is ticked.

default_thai_company/hooks.py

@@ -43,7 +43,7 @@ required_apps = ["erpnext"]
 # page_js = {"page" : "public/js/file.js"}
 
 # include js in doctype views
-# doctype_js = {"doctype" : "public/js/doctype.js"}
+doctype_js = {"Payment Entry": "public/js/payment_entry.js"}
 # doctype_list_js = {"doctype" : "public/js/doctype_list.js"}
 # doctype_tree_js = {"doctype" : "public/js/doctype_tree.js"}
 # doctype_calendar_js = {"doctype" : "public/js/doctype_calendar.js"}
@@ -90,7 +90,16 @@ fixtures = [
 	{"doctype": "Print Format", "filters": [["name", "in", ["Default Standard Sales Invoice"]]]},
 	{
 		"doctype": "Property Setter",
-		"filters": [["name", "in", ["Sales Invoice-main-default_print_format"]]],
+		"filters": [
+			[
+				"name",
+				"in",
+				[
+					"Sales Invoice-main-default_print_format",
+					"Payment Entry-apply_tax_withholding_amount-depends_on",
+				],
+			]
+		],
 	},
 	{"doctype": "Tax Withholding Category", "filters": [["name", "like", "WHT %"]]},
 ]
@@ -145,9 +154,9 @@ fixtures = [
 # ---------------
 # Override standard doctype classes
 
-# override_doctype_class = {
-# 	"ToDo": "custom_app.overrides.CustomToDo"
-# }
+override_doctype_class = {
+	"Sales Invoice": "default_thai_company.tax_withholding.ThaiSalesInvoice",
+}
 
 # Document Events
 # ---------------
@@ -164,6 +173,9 @@ doc_events = {
 	"Company": {
 		"on_update": "default_thai_company.tax_withholding.setup_company",
 	},
+	"Payment Entry": {
+		"validate": "default_thai_company.tax_withholding.set_customer_withholding",
+	},
 }
 
 # Scheduled Tasks