Fix shutdown UX: nohup wrapper prevents SSH session drop during host shutdown

When the host shuts down, sshd is killed before sending exit-status,
causing Go ssh library to return "wait: remote command exited without
exit status" — treated as failure even though the shutdown worked.

Changes:
- shutdown.go: wrap shutdown-like commands with nohup so the SSH
  session exits cleanly before sshd is killed by shutdown
- IsShutdownCommand() helper detects shutdown/poweroff/halt/reboot commands
- handlers_machines.go: classify expected shutdown-side-effect errors
  (no exit status, connection refused/reset) as success so the UI
  shows a green toast instead of a false error

internal/api/handlers_machines.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"strings"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -282,7 +283,36 @@ func (h *MachineHandler) Shutdown(w http.ResponseWriter, r *http.Request) {
 		writeJSON(w, ShutdownResponse{Success: false, Error: err.Error()})
 		return
 	}
-	writeJSON(w, ShutdownResponse{Success: result.Success, Output: result.Output, Error: result.Error})
+
+	responseSuccess := result.Success
+	responseOutput := result.Output
+	responseError := result.Error
+
+	if !responseSuccess && m.ShutdownCommand != "" && sshmanager.IsShutdownCommand(m.ShutdownCommand) {
+		if isExpectedShutdownError(result.Error) {
+			responseSuccess = true
+			responseError = ""
+			if responseOutput == "" {
+				responseOutput = "shutdown command sent (host session terminated as expected)"
+			}
+		}
+	}
+
+	writeJSON(w, ShutdownResponse{
+		Success: responseSuccess,
+		Output:  responseOutput,
+		Error:   responseError,
+	})
+}
+
+func isExpectedShutdownError(errMsg string) bool {
+	if errMsg == "" {
+		return false
+	}
+	return strings.Contains(errMsg, "remote command exited without exit status") ||
+		strings.Contains(errMsg, "connection refused") ||
+		strings.Contains(errMsg, "connection reset by peer") ||
+		strings.Contains(errMsg, "use of closed network connection")
 }
 
 func (h *MachineHandler) TestConnection(w http.ResponseWriter, r *http.Request) {