Allow PO template selection at sub-project level (or at time of PO creation)

Current behaviour:
The PO template can only be set in two places: as a company-wide default (Company Settings) or at a Project-Level within Budget & Fee Settings. There's no way to set it at sub-project/detailed classification level, or to choose it manually when raising a PO.

Use case:
Clients with two companies, each with its own PO template, sometimes run projects that cover both companies. That split is captured in the detailed classification at sub-project level. Since PO templates are only set at project level, there's no way to apply the correct PO template per sub-project.

Suggested solutions:

1. Allow the PO template to be set under Detailed Classification at the sub-project level, or

2. Allow the user to select which PO template to use at the point of capturing/creating the PO.