Introduce new classes and records in the `EnvelopeGenerator.API.Models` namespace to handle various functionalities:
- Add `Auth` record for managing authentication codes.
- Introduce `ContactLink` class for hyperlink management.
- Add `Culture` and `Cultures` classes for language and culture info.
- Implement `CustomImages` class for image management.
- Add `EnvelopeReceiverLogin` record for login requests.
- Introduce `ErrorViewModel` for error representation.
- Add `Image` class for image source and CSS management.
- Implement `Login` record for user authentication.
- Add `MainViewModel` with a nullable `Title` property.
- Introduce PDF annotation classes in `PsPdfKitAnnotation` namespace.
- Add `TFARegParams` class for 2FA registration parameters.
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