Key lessons learned:
1. Directory case sensitivity - app/Filament/ not app/filament/
2. UnitEnum/BackedEnum are PHP 8.2 built-in types, not classes
3. Always verify project structure before making changes
4. Learn from working code instead of assumptions
5. Stop and diagnose thoroughly when issues repeat
Change all Resources' navigationGroup from 'string|UnitEnum|null' to 'UnitEnum|string|null'
Union type order matters: UnitEnum first, then string
All 4 Resources updated:
- CategoryResource
- BrandResource
- PurchaseOrderResource
- StaffScheduleResource
- InventoryTransactionResource
PHP syntax check passed for all files
