chore: rewrite user-facing JS messages in Subcontracting module

Conservative cleanup of frappe.throw/msgprint messages per the message style
guide; meaning, severity, and .format() arguments are unchanged:

- index bare {} placeholders as {0}/{1}/... so translators can reorder
- move f-strings / .format() / concatenation out of _() (they break gettext
  extraction and never translate)
- wrap translatable dynamic values (DocType/Select labels) in _()
- fix grammar and colloquialisms
- drop no-op _() wrapping runtime-built strings

Part of #53976.

erpnext/subcontracting/doctype/subcontracting_receipt/subcontracting_receipt.js

@@ -143,7 +143,7 @@ frappe.ui.form.on("Subcontracting Receipt", {
 					if (!frm.doc.supplier) {
 						frappe.throw({
 							title: __("Mandatory"),
-							message: __("Please Select a Supplier"),
+							message: __("Please select a supplier"),
 						});
 					}