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/config/mod.rs

@@ -39,6 +39,7 @@ pub struct CollectorConfig {
     pub smart: SmartConfig,
     pub backup: BackupConfig,
     pub network: NetworkConfig,
+    pub nixos: NixOSConfig,
 }
 
 /// CPU collector configuration
@@ -113,6 +114,13 @@ pub struct SmartConfig {
     pub wear_critical_percent: f32,
 }
 
+/// NixOS collector configuration
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct NixOSConfig {
+    pub enabled: bool,
+    pub interval_seconds: u64,
+}
+
 /// Backup collector configuration
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct BackupConfig {