Add UI support organization and multi-facility calculation #34
Description
Replace the existing Facilities multi-select in the header with a two-tier grouped dropdown:
- Top tier: Organization names (as group headers with checkboxes)
- Nested tier: Facilities belonging to each organization
- Selecting an organization checkbox selects all its member facilities
- Deselecting an organization checkbox deselects all its member facilities
- Individual facilities can still be toggled independently