Adjust frontend to Email Service and new BackendURL

[Darjusch]

We should not hardcode the URL but instead get it from the Github secrets. - Its also the wrong URL right now.
New URL - https://api.kitaplatz-zentrale.de/
( Might also want to change to node 18 )

change /anmeldung to /signup

adjust the body according to the agreed interface:

Example:

{
  "email": "darjusch.schrand@code.berlin",
  "kitaId": "kitaid",
  "kitaName": "Kita A",
  "kitaDesiredAvailability": "May",
}

Example:

{
      "email": "darjusch.schrand@code.berlin",
      "fullAddress": "123 Main St, City",
      "desiredStartingMonth": "September",
      "actualOrExpectedBirthMonth": "June",
    }