Implement real-time terminal popup for system rebuild operations

- Add terminal popup UI component with 80% screen coverage and terminal styling
- Extend ZMQ protocol with CommandOutputMessage for streaming output
- Implement real-time output streaming in agent system rebuild handler
- Add keyboard controls (ESC/Q to close, ↑↓ to scroll) for popup interaction
- Fix system panel Build display to show actual NixOS build instead of config hash
- Update service filters in README with wildcard patterns for better matching
- Add periodic progress updates during nixos-rebuild execution
- Integrate command output handling in dashboard main loop

dashboard/src/ui/widgets/system.rs

@@ -422,9 +422,19 @@ impl SystemWidget {
             Span::styled("NixOS:", Typography::widget_title())
         ]));
         
+        let build_text = self.nixos_build.as_deref().unwrap_or("unknown");
+        lines.push(Line::from(vec![
+            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!("Build: {}", config_text), Typography::secondary())
+            Span::styled(format!("Config: {}", config_text), Typography::secondary())
+        ]));
+        
+        let users_text = self.active_users.as_deref().unwrap_or("unknown");
+        lines.push(Line::from(vec![
+            Span::styled(format!("Active users: {}", users_text), Typography::secondary())
         ]));
         
         let agent_hash_text = self.agent_hash.as_deref().unwrap_or("unknown");