Pickings back to draft

This module adds a way to recover transfers (pickings) that were canceled by mistake or that need to be reworked.

On the transfer form, a "Back to draft" button appears next to the Cancel button, but only when the transfer is in the canceled state. Any internal user can see and use it.

When you click it, the transfer and all of its stock moves are set back to the draft state, so the transfer becomes editable and can be confirmed and processed again.

A safety rule is enforced: the action only works on canceled records. If any of the moves involved is not canceled, Odoo stops the operation and shows an error message saying that only canceled moves can be set back to draft.

The module touches the transfer (stock.picking) and stock move (stock.move) models and only adds this button to the standard transfer form view; it does not change any other screen, report or automatic behaviour.