Add Country-Based Filtering for White-Labeler Dashboard and Assignment for Sales Agents

Since the option to add Sales Agents is already implemented, it would be very helpful to extend this with country-based assignment and filtering.

Specifically:

In the main customers overview / white‑labeler dashboard, it should be possible to filter customers by country.

Sales Agents should be assignable not only to specific customers, but also to a specific country (or selected countries), so that they automatically focus on customers from those regions.