Implement user authentication with email and password

[The Figma document](https://www.figma.com/file/uLiqrlxmOB5xCppIzmiUXV/Teacher-Workout?node-id=0%3A1) contains the entire UI of the project.

You can export any image (asset) directly from Figma. [Here's how](https://drive.google.com/file/d/1KJncuAGFvKEo6zrjC9H3O59auITE0q2h/view).

**Please do not try to recreate the UI design exactly.** Prioritize using de default UI mobile components and following the [Human Interface Guidelines](https://developer.apple.com/design/human-interface-guidelines/ios). The UI objects should be positioned roughly the same as in the Figma screens.

[The mobile flow can be found in the project Wiki](https://github.com/code4romania/teacher-workout-backend/wiki/Teacher-Workout-Mobile-flow).

_Please define related colors and text & button styles in the corresponding values files. Please extract all text values in the corresponding Localizable files so we can easily adjust them. <3_

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**Before starting working on this ticket, we need to check the API is ready and have the sign-in mutation deployed.**

We have the UI implemented for the Sign In screen, and we need to authenticate the user using our backend services.

After a successful login, the user is redirected to:

* the on-boarding process, if they have just created an account and are logging in for the first time; the user should always be redirected to the first onboarding screen after logging in if they have not yet gone through all of the onboarding screens
* their [Home screen](https://drive.google.com/file/d/1c8ONN6GTeeSGMX9U-wlHfQicNm3T9q6V/view?usp=sharing) if they have already finished the onboarding process


Start by checking that the backend API and sign-in mutation are deployed, then inspect the existing Sign In screen and consult the mobile flow in the project Wiki and the linked Figma document. Done means successful authentication routes users to the first incomplete onboarding screen or to Home when onboarding is complete, with styles and localized text kept in the corresponding values and Localizable files.

Written by the indexing model from the issue text.