fix(sales): load sales_classification.js via doctype_js, not app_include_js

app_include assets are appended to the desk's `Link: rel=preload` response
header. This deployment sits behind an edge proxy with a 4 KB header buffer;
the extra entry tipped the header over the limit → 502 "upstream sent too
big header" on every desk request (only at the edge proxy, so it was
invisible to local nginx/gunicorn curl checks).

Move the checkbox mutual-exclusion script to doctype_js for Sales Invoice
and Sales Order — loaded per-form, never added to the preload header. Same
mechanism already documented in jey_layout/hooks.py.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

jey_erp/hooks.py

@@ -15,7 +15,6 @@ app_include_js = [
     "/assets/jey_erp/js/sales_invoice_tax_articles.js",
     "/assets/jey_erp/js/sales_invoice_certificates.js",
     "/assets/jey_erp/js/sales_order_vat.js",
-    "/assets/jey_erp/js/sales_classification.js",
     "/assets/jey_erp/js/asset.js",
     "/assets/jey_erp/js/link_field_fix.js",
     "bank_excel_import.bundle.js",
@@ -27,7 +26,11 @@ app_include_css = [
 ]
 
 doctype_js = {
-    "Sales Invoice": "public/js/sales_invoice.js",
+    # sales_classification.js loaded via doctype_js (NOT app_include_js) so it
+    # does NOT bloat the desk's `Link: rel=preload` header — the edge proxy has a
+    # 4 KB header buffer and overflowing it returns 502. See jey_layout/hooks.py.
+    "Sales Invoice": ["public/js/sales_invoice.js", "public/js/sales_classification.js"],
+    "Sales Order": "public/js/sales_classification.js",
     "Purchase Invoice": "public/js/purchase_invoice.js",
     "Employee": "public/js/employee.js",
     "Bank Reconciliation Tool": "public/js/bank_reconciliation_tool.js",