Define typed Dart models for OrganizationUnit

Follow the existing project model conventions — inspect neighboring model files before writing to match constructor style (named parameters, required keyword), file naming, and directory placement. Use Dart's const constructor where all fields are final for compile-time immutability. For equality and hashCode, using only id is intentional and consistent with entity identity semantics. For OrganizationUnitTree.fromFlatList, build a Map keyed by id first, then iterate once to attach each node to its parent's children list — this is O(n) vs the naive O(n²) approach.

The root of the tree is the node with parentId == null. Handle the edge case where multiple roots exist (should not occur in valid data, but log a warning rather than throwing). DateTime parsing from Supabase returns ISO 8601 strings with timezone — use DateTime.parse() which handles this correctly. Do not use code generation (json_serializable, freezed) unless the rest of the project already uses it — hand-write serialization to match existing patterns.

Write unit tests using flutter_test. Test file location: test/models/organization_unit_test.dart. Scenarios: (1) fromJson round-trip with all fields populated. (2) fromJson with deletedAt = null.

(3) fromJson with an unknown unit_type string throws ArgumentError. (4) toJson produces correct snake_case keys. (5) copyWith produces a new instance with updated fields, original unchanged. (6) Two OrganizationUnit instances with the same id are == regardless of other field differences.

(7) OrganizationUnitTree.fromFlatList with 1 national + 3 regions + 9 chapters builds correct tree structure. (8) findById returns correct node for leaf, intermediate, and root. (9) findById returns null for non-existent id. Aim for 100% branch coverage on model logic.