Fix MachineResponse: expose last_seen_at/created_at, clean status string

The status field was being decorated with "(last seen ...)" suffix
which broke frontend statusVariant() matching and prevented the
LastSeen column from showing the separate timestamp.

Changes:
- machineToResp() now returns clean status ("online"/"offline")
- MachineResponse includes last_seen_at and created_at as separate fields
- Fix whitespace typo in WakeCheckIntervalSeconds field

internal/api/dto.go

@@ -28,6 +28,8 @@ type MachineResponse struct {
 	FingerprintConfirmed        bool    `json:"fingerprint_confirmed"`
 	HostKeyFingerprint         *string `json:"host_key_fingerprint"`
 	Status                      string  `json:"status"`
+	LastSeenAt                  *string `json:"last_seen_at"`
+	CreatedAt                   string  `json:"created_at"`
 }
 
 type TestConnectionResponse struct {