Pricing books: add multiple companies at once (multi-select + include child companies)

Description: Currently, pricing books require companies to be added individually. We need the ability to add multiple companies at once, either through multi-select or by including child companies automatically when a parent company is selected.

Use case: When setting up a pricing book for a parent company with a large number of child companies, we're forced to add each one individually. This becomes impractical at scale — a parent company structure with hundreds or thousands of child entities would require that many separate actions to configure pricing.

Proposed solution:

• Add a toggle to "Include child companies" when selecting a parent company in a pricing book.
• Enable multi-select in the company dropdown so multiple companies can be added to a pricing book in a single action.

Expected benefit: This would dramatically reduce the time and effort required to configure pricing books for large corporate structures — from hundreds or thousands of individual additions to a setup of minutes.