Update NixOS display format to show build hash and timestamp

- Change from showing version to build format: 'hash dd/mm/yy H:M:S'
- Parse nixos-version output to extract short hash and format date
- Update system widget to display 'Build:' instead of 'Version:'
- Remove version/build_date fields in favor of single build string
- Follow TODO.md specification for NixOS section layout

dashboard/src/ui/widgets/system.rs

@@ -14,8 +14,7 @@ use crate::ui::theme::{StatusIcons, Typography};
 #[derive(Clone)]
 pub struct SystemWidget {
     // NixOS information
-    nixos_version: Option<String>,
-    nixos_build_date: Option<String>,
+    nixos_build: Option<String>,
     active_users: Option<String>,
     
     // CPU metrics
@@ -63,8 +62,7 @@ struct StorageDrive {
 impl SystemWidget {
     pub fn new() -> Self {
         Self {
-            nixos_version: None,
-            nixos_build_date: None,
+            nixos_build: None,
             active_users: None,
             cpu_load_1min: None,
             cpu_load_5min: None,
@@ -301,14 +299,9 @@ impl Widget for SystemWidget {
         for metric in metrics {
             match metric.name.as_str() {
                 // NixOS metrics
-                "system_nixos_version" => {
-                    if let MetricValue::String(version) = &metric.value {
-                        self.nixos_version = Some(version.clone());
-                    }
-                }
-                "system_nixos_build_date" => {
-                    if let MetricValue::String(date) = &metric.value {
-                        self.nixos_build_date = Some(date.clone());
+                "system_nixos_build" => {
+                    if let MetricValue::String(build) = &metric.value {
+                        self.nixos_build = Some(build.clone());
                     }
                 }
                 "system_active_users" => {
@@ -390,9 +383,9 @@ impl Widget for SystemWidget {
             Span::styled("NixOS:", Typography::widget_title())
         ]));
         
-        let version_text = self.nixos_version.as_deref().unwrap_or("unknown");
+        let build_text = self.nixos_build.as_deref().unwrap_or("unknown");
         lines.push(Line::from(vec![
-            Span::styled(format!("Version: {}", version_text), Typography::secondary())
+            Span::styled(format!("Build: {}", build_text), Typography::secondary())
         ]));
 
         let users_text = self.active_users.as_deref().unwrap_or("unknown");