feat(statements): add Customer list "Generate Statements" button

Register doctype_list_js for Customer and add customer_list.js, which
lists customers with overdue invoices in a selection dialog (checkbox
table + select-all), then calls generate_statements and opens the
printable, one-page-per-customer document in a new window.

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

ns_app/hooks.py

@@ -15,6 +15,11 @@ doctype_js = {
     "Sales Invoice": "public/js/sales_invoice.js"
 }
 
+# Load on Customer list view (adds "Generate Statements" action)
+doctype_list_js = {
+    "Customer": "public/js/customer_list.js"
+}
+
 # Fixtures tracked in Git
 fixtures = [
     {