Implement remote command execution and visual feedback for service control

This implements the core functionality for executing remote commands through
the dashboard and providing real-time visual feedback to users.

Key Features:
- Remote service control (start/stop/restart) via existing keyboard shortcuts
- System rebuild command with maintenance mode integration
- Real-time visual feedback with service status transitions
- ZMQ command protocol extension for service and system operations

Implementation Details:
- Extended AgentCommand enum with ServiceControl and SystemRebuild variants
- Added agent-side handlers for systemctl and nixos-rebuild execution
- Implemented command status tracking system for visual feedback
- Enhanced services widget to show progress states (⏳ restarting)
- Integrated command execution with existing keyboard navigation

Keyboard Controls:
- Services Panel: Space (start/stop), R (restart)
- System Panel: R (nixos-rebuild switch)
- Backup Panel: B (trigger backup)

Technical Architecture:
- Command flow: UI → Dashboard → ZMQ → Agent → systemctl/nixos-rebuild
- Status tracking: InProgress/Success/Failed states with visual indicators
- Maintenance mode: Automatic /tmp/cm-maintenance file management
- Service feedback: Icon transitions (● → ⏳ → ● with status text)

agent/src/communication/mod.rs

@@ -99,4 +99,22 @@ pub enum AgentCommand {
     ToggleCollector { name: String, enabled: bool },
     /// Request status/health check
     Ping,
+    /// Control systemd service
+    ServiceControl {
+        service_name: String,
+        action: ServiceAction,
+    },
+    /// Rebuild NixOS system
+    SystemRebuild {
+        nixos_path: String, // Path to nixosbox directory
+    },
+}
+
+/// Service control actions
+#[derive(Debug, Clone, serde::Deserialize, serde::Serialize)]
+pub enum ServiceAction {
+    Start,
+    Stop,
+    Restart,
+    Status,
 }