feat: default Shipping Rule per Thai company

"Shipping Charges - <abbr>": Selling, Fixed, posting to a "Shipping
Charges" income account under Direct Income (created if the chart lacks
it) with the default cost center. Shipping Rule is named by its label,
so the company abbreviation keeps one rule per company, as ERPNext
names its tax templates. Created on Company save, after install, and by
a patch for existing companies.

The amount is entered on the transaction, not the rule: ERPNext
re-applies the rule on every recalculation and would reset the charge
row to the rule's fixed amount. ThaiShippingRule overrides the doctype
class so a Fixed rule with no amount only seeds the row and leaves the
entered amount alone.

default_thai_company/hooks.py

@@ -139,8 +139,12 @@ fixtures = [
 # ------------
 
 # before_install = "default_thai_company.install.before_install"
-# Fixtures are synced before this runs; VAT schemes for companies that already exist.
-after_install = "default_thai_company.vat.setup_companies"
+# Fixtures are synced before this runs; VAT schemes and Shipping Rules for companies
+# that already exist.
+after_install = [
+	"default_thai_company.vat.setup_companies",
+	"default_thai_company.shipping.setup_companies",
+]
 # Fixture import re-inserts the Location tree; restore lft/rgt for user-added nodes.
 after_migrate = "default_thai_company.assets.rebuild_locations"
 
@@ -191,6 +195,7 @@ after_migrate = "default_thai_company.assets.rebuild_locations"
 override_doctype_class = {
 	"Sales Invoice": "default_thai_company.tax_withholding.ThaiSalesInvoice",
 	"Payment Entry": "default_thai_company.tax_withholding.ThaiPaymentEntry",
+	"Shipping Rule": "default_thai_company.shipping.ThaiShippingRule",
 }
 
 # Document Events
@@ -213,6 +218,7 @@ doc_events = {
 			"default_thai_company.tax_withholding.setup_company",
 			"default_thai_company.vat.setup_company",
 			"default_thai_company.assets.setup_company",
+			"default_thai_company.shipping.setup_company",
 		],
 	},
 	"Payment Entry": {