Improve loading screen

[pdelboca]

New loading screen

The goal is to implement a loading/splash screen as shown below. The welcoming screen will be implemented in #453 so the scope of this ticket is:

• Implement the new blue loading screen
• Move the backend logic that checks that the FastAPI server initialization has completed from the Application module to the Desktop module.

ODE.-.Subsequent.runs.mp4

Technical Details

We already have an codebase aligned with this idea, however we need to refactor some things a little bit. We currently have 2 steps:

1. Start the Electron Application + FastAPI Server. (Handled in the desktop module.)
2. Run the application and waits until FastAPI server responds. (Handled in the Application module).

As I mentioned in #446 we need to migrate the logic that waits for the server so we only redirects to the application once the FastAPI server is fully loaded.

In terms of implementation, I suggest to refactor the current logic implemented in the onStart method of the store, extract the waiting for the server and move it to the desktop module. Leaving in the onStart method all the logic to load the configuration and stuff.

[romicolman]

Please, check issue #445 before implementing changes. Faith added additional info in comments.