fix: timezone-aware datetime for payment confirmation button

[Copilot]

Pressing "Potwierdź płatność" failed with Invalid datetime: missing timezone information because datetime.now() produces a naive datetime, which HA's DateTimeEntity rejects.

Changes

• coordinator.py — swap datetime.now() for dt_util.now() (homeassistant.util.dt), which always returns a timezone-aware datetime respecting HA's configured timezone:

  # before
  self._payment_confirmed_date = datetime.now().isoformat()
  # after
  self._payment_confirmed_date = dt_util.now().isoformat()

• datetime.py — harden native_value to handle legacy naive ISO strings already persisted in storage by attaching UTC if tzinfo is absent:

  dt = datetime.fromisoformat(raw)
  if dt.tzinfo is None:
      dt = dt.replace(tzinfo=timezone.utc)
  return dt