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/api/dto.go

@@ -11,6 +11,7 @@ type MachineRequest struct {
 	BroadcastAddr               *string `json:"broadcast_addr"`
 	WakeTimeoutSeconds          int     `json:"wake_timeout_seconds"`
 	WakeCheckIntervalSeconds    int     `json:"wake_check_interval_seconds"`
+	ShutdownCommand             string  `json:"shutdown_command,omitempty"`
 }
 
 type MachineResponse struct {
@@ -30,13 +31,20 @@ type MachineResponse struct {
 	Status                      string  `json:"status"`
 	LastSeenAt                  *string `json:"last_seen_at"`
 	CreatedAt                   string  `json:"created_at"`
+	ShutdownCommand             string  `json:"shutdown_command"`
 }
 
 type TestConnectionResponse struct {
-	Success    bool    `json:"success"`
-	Output     string  `json:"output,omitempty"`
-	Error      string  `json:"error,omitempty"`
-	Fingerprint string `json:"fingerprint,omitempty"`
+	Success     bool    `json:"success"`
+	Output      string  `json:"output,omitempty"`
+	Error       string  `json:"error,omitempty"`
+	Fingerprint string  `json:"fingerprint,omitempty"`
+}
+
+type ShutdownResponse struct {
+	Success bool   `json:"success"`
+	Output  string `json:"output,omitempty"`
+	Error   string `json:"error,omitempty"`
 }
 
 type SyncPairRequest struct {