Implement Project Mapping Logic

**Description:** 
Map donations to specific projects using project_id.

**Tasks:** 
- \[ \] Validate project_id format 
- \[ \] Group donations by project_id 
- \[ \] Store mapping in contract storage 
- \[ \] Handle missing project_id 
- \[ \] Test mapping logic

**Acceptance Criteria:** 
- Valid project_id mapped correctly
- Invalid IDs rejected 
- Donations grouped properly
