Add waiting screen to validate and centralise CentralWaitingScreen

[AdityaGupta716]

Description

What does this PR do?

• Adds a waiting screen when validating with include_central=True and the connection method is not local_filesystem (i.e. ssh, aws, gdrive), where the connection can be slow
• Centralises the waiting screen logic by replacing SearchingCentralForNextSubSesPopup and ValidatingCentralPopup with a single reusable CentralWaitingScreen(message: str) class

Why is this PR needed?

Fixes #604

What is the approach?

• CentralWaitingScreen takes a message string, displays a Label + LoadingIndicator, consistent with the pattern established in Suggest next sub ses remote #484 and ADD: Workers for transferring data and Loading Animation #479
• Validation runs in a @work(exclusive=True, thread=True) thread, UI updates use call_from_thread, matching the existing sub/ses suggestion pattern

Is this a breaking change?

No

How has this PR been tested?

All 65 TUI tests pass locally.

Checklist

• The code has been tested locally
• Tests have been added to cover all new functionality
• The code has been formatted with pre-commit

[JoeZiminski]

Thanks @AdityaGupta716! This is a nice feature addition, I will try and take a look at this over the next few weeks. Thanks for your contribution!