feat: VM/CT Start/Stop im Frontend + Agent proxmox_action Handler

- Agent: neuer 'proxmox_action' Command (start/stop/shutdown fuer VMs und LXC)
- Backend: POST /api/v1/agents/{id}/proxmox/action Endpoint
- Frontend: Start/Stop/Shutdown Buttons in VMs- und Container-Tab
  - Play (gruen) = starten (nur wenn gestoppt)
  - Power (gelb) = Graceful Shutdown (nur VMs, nur wenn running)
  - Stop (rot) = Hard Stop (wenn running)
- api/client.js: proxmoxAction() Methode

agent-linux/ws/handler.go

@@ -47,6 +47,8 @@ func (h *Handler) HandleCommand(msg Message) {
 		response = h.handleUpdate(msg)
 	case "zpoolscrub":
 		response = h.handleZpoolScrub(msg)
+	case "proxmox_action":
+		response = h.handleProxmoxAction(msg)
 	case "pty_start":
 		response = h.handlePTYStart(msg)
 	case "pty_stop":
@@ -481,3 +483,65 @@ func (h *Handler) handleZpoolScrub(msg Message) Message {
 	}
 	return response
 }
+
+// handleProxmoxAction: VM/CT starten oder stoppen via pvesh
+func (h *Handler) handleProxmoxAction(msg Message) Message {
+	response := Message{Type: "response", ID: msg.ID}
+
+	vmType, _ := msg.Params["type"].(string)   // "vm" oder "ct"
+	action, _ := msg.Params["action"].(string)  // "start", "stop", "shutdown"
+	vmidFloat, ok := msg.Params["vmid"].(float64)
+	if !ok {
+		response.Status = "error"
+		response.Error = "Parameter 'vmid' erforderlich"
+		return response
+	}
+	vmid := int(vmidFloat)
+
+	if vmType != "vm" && vmType != "ct" {
+		response.Status = "error"
+		response.Error = "Parameter 'type' muss 'vm' oder 'ct' sein"
+		return response
+	}
+	if action != "start" && action != "stop" && action != "shutdown" {
+		response.Status = "error"
+		response.Error = "Parameter 'action' muss 'start', 'stop' oder 'shutdown' sein"
+		return response
+	}
+
+	// Node-Name ermitteln
+	nodeName, _ := os.Hostname()
+	if nodeName == "" {
+		nodeName = "localhost"
+	}
+
+	// pvesh-Pfad aufbauen
+	var apiPath string
+	if vmType == "vm" {
+		apiPath = fmt.Sprintf("/nodes/%s/qemu/%d/status/%s", nodeName, vmid, action)
+	} else {
+		if action == "shutdown" {
+			action = "stop" // LXC kennt kein "shutdown", stop reicht
+		}
+		apiPath = fmt.Sprintf("/nodes/%s/lxc/%d/status/%s", nodeName, vmid, action)
+	}
+
+	log.Printf("Proxmox Action: pvesh create %s", apiPath)
+
+	cmd := exec.Command("/usr/bin/pvesh", "create", apiPath, "--output-format", "json")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Printf("Proxmox Action Fehler: %v — %s", err, string(output))
+		response.Status = "error"
+		response.Error = fmt.Sprintf("pvesh fehlgeschlagen: %v — %s", err, strings.TrimSpace(string(output)))
+		return response
+	}
+
+	log.Printf("Proxmox Action erfolgreich: %s %s %d", action, vmType, vmid)
+	response.Status = "ok"
+	response.Data = map[string]interface{}{
+		"message": fmt.Sprintf("%s %d: %s ausgeführt", vmType, vmid, action),
+		"output":  strings.TrimSpace(string(output)),
+	}
+	return response
+}