[FR-12] Add dashboard filtering and drill down

[manuelgrabmayer]

Requirement:
The dashboard shall support filtering and drill-down by region, sector, time range, donor, and recipient.

Goal:
Allow users to explore the dataset interactively instead of only seeing static summaries.

Acceptance criteria:

1. Users can filter by at least region and sector.
2. Users can filter by time range where date data exists.
3. Users can filter by donor or recipient where these fields are available.
4. Dashboard components update based on selected filters.
5. Empty filter results are handled with a clear message.