Point of Sale - No Alter Bank statement lines of Pos Orders

This module protects the accounting entries created by the Point of Sale. When a Point of Sale order reaches the paid, done or invoiced stage, the bank statement lines generated for it become locked.

• Users can no longer delete those bank statement lines: Odoo shows an error message explaining that lines linked to paid, done or invoiced orders cannot be removed.
• Users can no longer modify the amount or the customer (partner) on those lines: any attempt is blocked with a similar error message.
• Other fields on the bank statement line are not restricted by this module; only the amount and the customer are protected.

The rule is applied automatically whenever someone tries to save a change or delete a line, so no manual setup or menu is needed. It works on the account.bank.statement.line model, which is the same screen used in Accounting for bank statements, and it relies on the link to the Point of Sale order (pos_statement_id) to know whether the order is paid, done or invoiced.

In practice, this keeps the accounting records consistent with the Point of Sale orders and avoids discrepancies between the till session and the bank statement. It is a safety fix for standard Odoo behaviour, aimed at accountants and Point of Sale managers who need the payment records to stay trustworthy.