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

dashboard/src/ui/widgets/mod.rs

@@ -5,11 +5,13 @@ pub mod backup;
 pub mod cpu;
 pub mod memory;
 pub mod services;
+pub mod system;
 
 pub use backup::BackupWidget;
 pub use cpu::CpuWidget;
 pub use memory::MemoryWidget;
 pub use services::ServicesWidget;
+pub use system::SystemWidget;
 
 /// Widget trait for UI components that display metrics
 pub trait Widget {