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/sshmanager/shutdown.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"strings"
 	"time"
 )
 
@@ -13,6 +14,20 @@ type ShutdownResult struct {
 	Error   string
 }
 
+func IsShutdownCommand(cmd string) bool {
+	c := strings.TrimSpace(strings.ToLower(cmd))
+	for _, p := range []string{
+		"shutdown", "poweroff", "halt", "reboot",
+		"sudo shutdown", "sudo poweroff", "sudo halt", "sudo reboot",
+		"systemctl poweroff", "systemctl halt", "systemctl reboot",
+	} {
+		if strings.HasPrefix(c, p) {
+			return true
+		}
+	}
+	return false
+}
+
 func RunRemoteCommand(ctx context.Context, host string, port int, user, privKeyPath, knownHostsPath string, strictHostKeyChecking bool, command string) (*ShutdownResult, error) {
 	conn, _, _, err := dialSSH(ctx, host, port, user, privKeyPath, knownHostsPath, strictHostKeyChecking)
 	if err != nil {
@@ -30,12 +45,20 @@ func RunRemoteCommand(ctx context.Context, host string, port int, user, privKeyP
 	session.Stdout = &stdout
 	session.Stderr = &stderr
 
+	effectiveCmd := command
+	if IsShutdownCommand(command) {
+		effectiveCmd = fmt.Sprintf(
+			"nohup %s >/dev/null 2>&1 </dev/null & sleep 1; echo shutdown_initiated",
+			command,
+		)
+	}
+
 	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
 	defer cancel()
 
 	done := make(chan error, 1)
 	go func() {
-		done <- session.Run(command)
+		done <- session.Run(effectiveCmd)
 	}()
 
 	select {
@@ -53,7 +76,7 @@ func RunRemoteCommand(ctx context.Context, host string, port int, user, privKeyP
 	case <-ctx.Done():
 		return &ShutdownResult{
 			Success: false,
-			Error:   "command timed out after 15 seconds (machine may be shutting down)",
+			Error:   "command timed out after 15 seconds",
 		}, nil
 	}
 }