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/metrics/mod.rs

@@ -7,7 +7,7 @@ use tracing::{debug, error, info};
 use crate::cache::MetricCacheManager;
 use crate::collectors::{
     backup::BackupCollector, cpu::CpuCollector, disk::DiskCollector, memory::MemoryCollector,
-    systemd::SystemdCollector, Collector,
+    nixos::NixOSCollector, systemd::SystemdCollector, Collector,
 };
 use crate::config::{AgentConfig, CollectorConfig};
 
@@ -100,6 +100,12 @@ impl MetricCollectionManager {
                     collectors.push(Box::new(backup_collector));
                     info!("Backup collector initialized");
                 }
+
+                if config.nixos.enabled {
+                    let nixos_collector = NixOSCollector::new(config.nixos.clone());
+                    collectors.push(Box::new(nixos_collector));
+                    info!("NixOS collector initialized");
+                }
             }
         }