Implement core server-side functionality for the warning system including:
- SyncTo: synchronizes connected players and their warnings to a client
- OpenFor: opens the warning menu for authorized players
- Chat commands: /warns, !warns, /warn to open the menu
- Net receivers for adding, removing, and clearing warnings
- Chat announcement system for new warnings with configurable scope (all, target, staff)
- Access control checks on all admin operations
