feat: Thai Tax Withholding Category fixtures with per-company accounts

Add 22 Tax Withholding Category fixtures covering Thailand domestic
withholding (Taw.Paw. 4/2528, Sec. 50(2), Sec. 70) with rates and
thresholds. Fixtures ship without accounts; on import each Thai company
gets "Withholding Tax Payable" (Liability) and "Withholding Tax
Receivable" (Asset) created and the payable account linked. Company
on_update does the same for companies created later. Site-configured
accounts survive fixture re-import on migrate.

default_thai_company/hooks.py

@@ -92,6 +92,7 @@ fixtures = [
 		"doctype": "Property Setter",
 		"filters": [["name", "in", ["Sales Invoice-main-default_print_format"]]],
 	},
+	{"doctype": "Tax Withholding Category", "filters": [["name", "like", "WHT %"]]},
 ]
 
 # Installation
@@ -156,7 +157,13 @@ doc_events = {
 	"Letter Head": {
 		"before_import": "default_thai_company.utils.stash_letter_head_source",
 		"before_insert": "default_thai_company.utils.restore_letter_head_source",
-	}
+	},
+	"Tax Withholding Category": {
+		"before_import": "default_thai_company.tax_withholding.prepare_fixture_accounts",
+	},
+	"Company": {
+		"on_update": "default_thai_company.tax_withholding.setup_company",
+	},
 }
 
 # Scheduled Tasks