fix: deploy keys modal crash on null/undefined arrays

- sshmanager/deploy.go: initialize DeployResult with empty slices
  instead of nil to prevent null serialization in JSON
- Machines.tsx: use ?? [] fallback for messages and errors arrays
  in deploy keys modal to handle null/undefined gracefully

internal/sshmanager/deploy.go

@@ -26,7 +26,7 @@ type DeployResult struct {
 }
 
 func DeployKeysToMachine(ctx context.Context, serverKeyPath, serverPubKey string, host string, port int, user string, keys []DeployKey, knownHostsHost string, addServerPubKey bool) (*DeployResult, error) {
-	result := &DeployResult{Success: true}
+	result := &DeployResult{Success: true, Messages: []string{}, Errors: []string{}}
 
 	addr := fmt.Sprintf("%s:%d", host, port)