Implement filtering by region

[sawaymas]

Description:

• Let the user narrow down the list of countries by region.

Tasks:

Create a region filter.

• Add an option to show all regions.
• Filter the fetched data without mutating the original data.
• Update the active country when the filter changes.
• Show a message if the filter produces an empty result.

Acceptance:

• The filter affects which countries can be displayed.
• The original data is not mutated.
• The application handles empty results.
• The active country is valid after filtering.