fix(column-mappings): make Standard Field optional

Auto-detect inserts a row for every header found in the sample file,
including ones it couldn't match against the synonym list — those rows
went in with an empty standard_field. With Standard Field marked
reqd=1, any subsequent action that triggered a save (Match accounts,
Create customers, etc. routed through the "save first?" prompt) failed
with "Mandatory fields required in table Column Mappings, Row N".

The reqd was already redundant: excel_parser skips column_mappings rows
without a standard_field, and Bank Statement Importer.validate enforces
the actual required-minimum set (Date / Reference + Debit/Credit or
Amount) at the parent level based on amount_mode. Dropping reqd from
the child lets unmapped rows coexist in the table for visibility.

jey_erp/jey_erp/doctype/bank_integration_excel_column_mapping/bank_integration_excel_column_mapping.json

@@ -19,12 +19,12 @@
    "description": "Column header text in the Excel file. Upload a Sample File above to pick from a list."
   },
   {
+   "description": "Leave blank for columns you don't want to use. The parser ignores rows without a Standard Field, but the row itself stays in the table for visibility.",
    "fieldname": "standard_field",
    "fieldtype": "Select",
    "in_list_view": 1,
    "label": "Standard Field",
-   "options": "\nDate\nReference Number\nCounterparty\nCounterparty Tax ID (VOEN)\nCounterparty IBAN\nAmount\nDebit\nCredit\nDirection\nCurrency\nPurpose\nDescription",
-   "reqd": 1
+   "options": "\nDate\nReference Number\nCounterparty\nCounterparty Tax ID (VOEN)\nCounterparty IBAN\nAmount\nDebit\nCredit\nDirection\nCurrency\nPurpose\nDescription"
   },
   {
    "fieldname": "notes",
@@ -35,7 +35,7 @@
  ],
  "istable": 1,
  "links": [],
- "modified": "2026-05-08 00:00:00.000000",
+ "modified": "2026-05-18 00:00:00.000000",
  "modified_by": "Administrator",
  "module": "Jey Erp",
  "name": "Bank Integration Excel Column Mapping",