Add configuration hash display to system panel

- Collect config hash from cloned nixos-config git repository
- Display "Config: xxxxx" after "Build: xxxxx" in NixOS section
- Uses /var/lib/cm-dashboard/nixos-config directory
- Shows actual configuration hash vs nixpkgs build hash

agent/src/collectors/nixos.rs

@@ -63,6 +63,29 @@ impl NixOSCollector {
         Ok("unknown".to_string())
     }
 
+    /// Get configuration hash from cloned nixos-config git repository
+    fn get_config_hash(&self) -> Result<String, Box<dyn std::error::Error>> {
+        // Get git hash from the cloned nixos-config directory
+        let config_path = "/var/lib/cm-dashboard/nixos-config";
+        
+        let output = Command::new("git")
+            .args(&["log", "-1", "--format=%h"])
+            .current_dir(config_path)
+            .output()?;
+        
+        if !output.status.success() {
+            return Err("git log command failed".into());
+        }
+        
+        let hash = String::from_utf8_lossy(&output.stdout).trim().to_string();
+        
+        if hash.is_empty() {
+            return Err("Empty git hash output".into());
+        }
+        
+        Ok(hash)
+    }
+
     /// Get currently active users
     fn get_active_users(&self) -> Result<Vec<String>, Box<dyn std::error::Error>> {
         let output = Command::new("who").output()?;
@@ -148,6 +171,31 @@ impl Collector for NixOSCollector {
             }
         }
 
+        // Collect config hash
+        match self.get_config_hash() {
+            Ok(hash) => {
+                metrics.push(Metric {
+                    name: "system_config_hash".to_string(),
+                    value: MetricValue::String(hash),
+                    unit: None,
+                    description: Some("NixOS configuration git hash".to_string()),
+                    status: Status::Ok,
+                    timestamp,
+                });
+            }
+            Err(e) => {
+                debug!("Failed to get config hash: {}", e);
+                metrics.push(Metric {
+                    name: "system_config_hash".to_string(),
+                    value: MetricValue::String("unknown".to_string()),
+                    unit: None,
+                    description: Some("Config hash (failed to detect)".to_string()),
+                    status: Status::Unknown,
+                    timestamp,
+                });
+            }
+        }
+
         // Collect agent hash
         match self.get_agent_hash() {
             Ok(hash) => {

dashboard/src/ui/widgets/system.rs

@@ -14,6 +14,7 @@ use crate::ui::theme::{StatusIcons, Typography};
 pub struct SystemWidget {
     // NixOS information
     nixos_build: Option<String>,
+    config_hash: Option<String>,
     active_users: Option<String>,
     agent_hash: Option<String>,
     
@@ -64,6 +65,7 @@ impl SystemWidget {
     pub fn new() -> Self {
         Self {
             nixos_build: None,
+            config_hash: None,
             active_users: None,
             agent_hash: None,
             cpu_load_1min: None,
@@ -322,6 +324,11 @@ impl Widget for SystemWidget {
                         self.nixos_build = Some(build.clone());
                     }
                 }
+                "system_config_hash" => {
+                    if let MetricValue::String(hash) = &metric.value {
+                        self.config_hash = Some(hash.clone());
+                    }
+                }
                 "system_active_users" => {
                     if let MetricValue::String(users) = &metric.value {
                         self.active_users = Some(users.clone());
@@ -418,6 +425,11 @@ impl SystemWidget {
             Span::styled(format!("Build: {}", build_text), Typography::secondary())
         ]));
 
+        let config_text = self.config_hash.as_deref().unwrap_or("unknown");
+        lines.push(Line::from(vec![
+            Span::styled(format!("Config: {}", config_text), Typography::secondary())
+        ]));
+
         let agent_hash_text = self.agent_hash.as_deref().unwrap_or("unknown");
         let short_hash = if agent_hash_text.len() > 8 && agent_hash_text != "unknown" {
             &agent_hash_text[..8]