fix(statements): restore Customer list button (merge listview_settings)

The list button was registered by reassigning
frappe.listview_settings['Customer'] in a globally-loaded script, but
ERPNext's own Customer list_js (loaded when the list opens) overwrote it,
so the button never appeared. Register it via doctype_list_js instead —
which Frappe appends after the doctype's own list_js — and merge into the
existing settings (wrapping onload, preserving ERPNext's add_fields)
rather than reassigning. The form button and shared ns_statements helpers
stay in customer_statements.js.

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

ns_app/hooks.py

@@ -16,6 +16,11 @@ doctype_js = {
     "Sales Invoice": "public/js/sales_invoice.js"
 }
 
+# Load on Customer list view (merges the "Generate Statements" action)
+doctype_list_js = {
+    "Customer": "public/js/customer_list.js"
+}
+
 # Ensure custom fields exist after every migrate
 after_migrate = "ns_app.setup.after_migrate"