feat: add dlv_account_move_copy_ref — copy customer ref on duplicate of customer invoice/credit note, with OK-popup warning

dlv_account_move_copy_ref/README.md

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+# dlv_account_move_copy_ref
+
+## Overview
+
+In core Odoo the reference of an invoice (`account.move.ref`, label
+**Customer Reference** on customer documents) is not copied when the invoice is
+duplicated. This addon makes it copy, and warns the user afterwards.
+
+## How it works
+
+- **Python** (`models/account_move.py`): `ref` gets `copy=True`. `copy_data()`
+  removes it again for every move that is not `out_invoice` / `out_refund`, so
+  vendor bills and journal entries keep the core behaviour. An explicit `ref`
+  in the `default` dict always wins (credit note reversal sets its own).
+- **JS** (`static/src/js/duplicate_warning.js`): patches
+  `FormController.duplicateRecord`. After the duplicate, if the new record is an
+  `account.move` customer invoice/credit note with a non-empty `ref`, a modal alert dialog (OK button)
+  warning is shown: "'Customer Reference' was also copied, check
+  whether it is still relevant or should be removed." (nl: "'Referentie klant'
+  werd ook gekopieerd, kijk na of dit relevant is of dient verwijderd te
+  worden.")
+
+## Scope
+
+- Only the Duplicate action in the form view shows the warning. Server-side
+  `copy()` calls (e.g. recurring entries) copy the reference silently.
+- No warning when `ref` is empty.
+
+## Compatibility
+
+- Odoo 16.0 (Community and Enterprise)
+- Depends on: `account`
+
+## Credits
+
+bv Domus La Vila — https://domuslavila.eu
+
+License: LGPL-3