feat(statements): bill late fee as a collectible Sales Invoice

Charge a late-payment fee when statements are generated so the customer's
receivable reflects it (the gap ERPNext Dunning leaves — it never
increases AR). The fee is billed as a submitted Sales Invoice (item ->
Dunning Type income account, rate = computed fee) rather than a Journal
Entry, so the app's existing payment flow (Run Payment / AutoPay /
multi-invoice) charges and settles it automatically via its Sales Invoice
references — a bare JE would sit uncollected.

Fee schedule/amounts come from the existing Dunning Type settings (yearly
rate_of_interest + flat dunning_fee), interest computed with ERPNext's own
Dunning formula. The fee Item is configured via a new Late Fee Item custom
field on Dunning Type (created in an after_migrate hook; ns_app/setup.py).
Nothing is auto-seeded: generation stops with a clear error if no Dunning
Type is configured or its income account / fee item is unset.

Billing is idempotent per customer/company/month, and prior fee invoices
are excluded from the interest base (no fee-on-fee). The fee invoice shows
on the statement flagged 'late fee', folded into Total Due (which equals
the customer's balance and is fully collectible).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ns_app/setup.py

@@ -0,0 +1,26 @@
+"""App setup: custom fields created/synced on migrate."""
+
+import frappe
+from frappe.custom.doctype.custom_field.custom_field import create_custom_fields
+
+# Fee schedule/amounts live on ERPNext's Dunning Type; this adds the one thing
+# it lacks — the Item used to bill a late fee as a Sales Invoice.
+CUSTOM_FIELDS = {
+    "Dunning Type": [
+        {
+            "fieldname": "custom_late_fee_item",
+            "label": "Late Fee Item",
+            "fieldtype": "Link",
+            "options": "Item",
+            "insert_after": "income_account",
+            "description": (
+                "Item used to bill a late-payment fee as a Sales Invoice when "
+                "customer statements are generated."
+            ),
+        }
+    ]
+}
+
+
+def after_migrate():
+    create_custom_fields(CUSTOM_FIELDS)