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

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]