Return `seq[ContactsDetails]` from `getContactsByGroup`

[osmaczko]

TLDR: the getContactsByGroup function in src/app_service/service/contacts/service.nim currently returns seq[ContactsDto], but that return value is most of the time used to get ContactsDetails and the original property is already ContactsDetails, so we're doing type transfers uselessly. So change the signature to return seq[ContactsDetails] directly

          Maybe return `seq[ContactsDetails]` from this function?

Originally posted by @igor-sirotin in #10805 (comment)

          This function is called with `getContacts` in `initContactRequestsModel`.
          And then for each contact we call `createItemFromPublicKey` which calls `getContactDetails`.

          So we do have `ContactDetails` in the beginning, then switch it to `ContactDto` and eventually back to `ContactDetails`.

Originally posted by @igor-sirotin in #10805 (comment)