Add shutdown machine feature with configurable command

Backend:
- New migration: add shutdown_command TEXT column to machines table
- Machine model updated with ShutdownCommand field (Create/GetAll/GetByID/Update)
- MachineRequest/MachineResponse DTOs updated with shutdown_command field
- New ShutdownResponse DTO
- New POST /api/machines/{id}/shutdown handler via SSH
- Refactor sshmanager/testconn.go: extract dialSSH() helper shared with shutdown.go
- New sshmanager/shutdown.go: RunRemoteCommand with 15s timeout

Frontend:
- New shutdownMachine() API helper and ShutdownResponse type in client.ts
- New shutdown_command field in MachineForm
- Power button (amber) in machines table actions
- Shutdown confirmation modal with WoL warning notice
- shutdown_command input field in machine edit/create form
- Machine interface updated with shutdown_command and last_seen_at fields

internal/sshmanager/shutdown.go

@@ -0,0 +1,59 @@
+package sshmanager
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"time"
+)
+
+type ShutdownResult struct {
+	Success bool
+	Output  string
+	Error   string
+}
+
+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 {
+		return &ShutdownResult{Success: false, Error: err.Error()}, nil
+	}
+	defer conn.Close()
+
+	session, err := conn.NewSession()
+	if err != nil {
+		return &ShutdownResult{Success: false, Error: fmt.Sprintf("session: %v", err)}, nil
+	}
+	defer session.Close()
+
+	var stdout, stderr bytes.Buffer
+	session.Stdout = &stdout
+	session.Stderr = &stderr
+
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+
+	done := make(chan error, 1)
+	go func() {
+		done <- session.Run(command)
+	}()
+
+	select {
+	case err := <-done:
+		if err != nil {
+			return &ShutdownResult{
+				Success: false,
+				Error:   fmt.Sprintf("exec: %v, stderr: %s", err, stderr.String()),
+			}, nil
+		}
+		return &ShutdownResult{
+			Success: true,
+			Output:  stdout.String(),
+		}, nil
+	case <-ctx.Done():
+		return &ShutdownResult{
+			Success: false,
+			Error:   "command timed out after 15 seconds (machine may be shutting down)",
+		}, nil
+	}
+}