Clear all partners in bank statement lines

This module adds a single action to bank statements in Odoo: a button labelled 'Clear partners'.

• The button appears on the bank statement form, right after the existing validate/action button.
• It is only visible when the statement is still open, has at least one line, and not all lines are already reconciled.
• When clicked, it goes through all the statement lines and, for each line that is not reconciled, it empties the partner field.
• Lines that are already reconciled keep their partner untouched.

This is useful when a statement was imported or entered with the wrong partner on many lines and you want to reset them in one go instead of editing each line manually. It only touches the partner information on unreconciled lines; amounts, dates, references and reconciliation status are not changed.

The module works on the account.bank.statement model and its lines, and only depends on the standard Accounting app. No new menus, reports, emails or scheduled jobs are added.