Implement unified system widget with NixOS info, CPU, RAM, and Storage

- Create NixOS collector for version and active users detection
- Add SystemWidget combining all system information in TODO.md layout
- Replace separate CPU/Memory widgets with unified system display
- Add tree structure for storage with drive temperature/wear info
- Support NixOS version, active users, load averages, memory usage
- Follow exact decimal formatting from specification

agent/src/collectors/mod.rs

@@ -7,6 +7,7 @@ pub mod cpu;
 pub mod disk;
 pub mod error;
 pub mod memory;
+pub mod nixos;
 pub mod systemd;
 
 pub use error::CollectorError;