Allow invoices to target either a client or a client group by making
`client_id` nullable and validating that exactly one recipient is set.
Load group relations in invoice data so the frontend can display group
details in invoice views and type definitions.
Make quote acceptance reuse an existing invoice instead of creating a
duplicate, and surface backend status update errors in the quote UI.
Add price list management across the API, store, services, routes,
navigation, and sales views.
Support quotes for either a client or a client group, including PDF
download and nullable client validation for group-based recipients.
Extend client groups to manage assigned clients directly from the form
and detail views, and refresh supplier, intervention, stock, and order
screens with updated interactions and layouts.
