Migrate service control from ZMQ to SSH with real-time progress

Replace ZMQ-based service start/stop commands with SSH execution in tmux
popups. This provides better user feedback with real-time systemctl output
while eliminating blocking operations from the main message processing loop.

Changes:
- Service start/stop now use SSH with progress display
- Added backup functionality with 'B' key
- Preserved transitional icons (↑/↓) for immediate visual feedback
- Removed all ZMQ service control commands and handlers
- Updated configuration to include backup_alias setting
- All operations (rebuild, backup, services) now use consistent SSH interface

This ensures stable heartbeat processing while providing superior user
experience with live command output and service status feedback.

dashboard/src/communication/mod.rs

@@ -5,40 +5,6 @@ use zmq::{Context, Socket, SocketType};
 
 use crate::config::ZmqConfig;
 
-/// Commands that can be sent to agents
-#[derive(Debug, Clone, serde::Deserialize, serde::Serialize)]
-pub enum AgentCommand {
-    /// Request immediate metric collection
-    CollectNow,
-    /// Change collection interval
-    SetInterval { seconds: u64 },
-    /// Enable/disable a collector
-    ToggleCollector { name: String, enabled: bool },
-    /// Request status/health check
-    Ping,
-    /// Control systemd service
-    ServiceControl {
-        service_name: String,
-        action: ServiceAction,
-    },
-    /// Rebuild NixOS system
-    SystemRebuild {
-        git_url: String,
-        git_branch: String,
-        working_dir: String,
-        api_key_file: Option<String>,
-    },
-}
-
-/// Service control actions
-#[derive(Debug, Clone, serde::Deserialize, serde::Serialize)]
-pub enum ServiceAction {
-    Start,
-    Stop,
-    Status,
-    UserStart,  // User-initiated start (clears user-stopped flag)
-    UserStop,   // User-initiated stop (marks as user-stopped)
-}
 
 /// ZMQ consumer for receiving metrics from agents
 pub struct ZmqConsumer {
@@ -202,42 +168,3 @@ impl ZmqConsumer {
     }
 }
 
-/// ZMQ command sender for sending commands to agents
-pub struct ZmqCommandSender {
-    context: Context,
-}
-
-impl ZmqCommandSender {
-    pub fn new(_config: &ZmqConfig) -> Result<Self> {
-        let context = Context::new();
-
-        info!("ZMQ command sender initialized");
-
-        Ok(Self { context })
-    }
-
-    /// Send a command to a specific agent
-    pub async fn send_command(&self, hostname: &str, command: AgentCommand) -> Result<()> {
-        // Create a new PUSH socket for this command (ZMQ best practice)
-        let socket = self.context.socket(SocketType::PUSH)?;
-
-        // Set socket options
-        socket.set_linger(1000)?; // Wait up to 1 second on close
-        socket.set_sndtimeo(5000)?; // 5 second send timeout
-
-        // Connect to agent's command port (6131)
-        let address = format!("tcp://{}:6131", hostname);
-        socket.connect(&address)?;
-
-        // Serialize command
-        let serialized = serde_json::to_vec(&command)?;
-
-        // Send command
-        socket.send(&serialized, 0)?;
-
-        info!("Sent command {:?} to agent at {}", command, hostname);
-
-        // Socket will be automatically closed when dropped
-        Ok(())
-    }
-}