cleanup GUI entrypoints and mocking

[Frizi]

Pull Request Description

This is a set of split off changes made as a side effect while working on engine reconnection handling.

Cleaned up GUI e2e setup, unified as much of the entrypoint code as possible. Currently the only real difference between the real and testing entrypoint is mocking of all network calls and not loading through dashboard.

I've managed to completely get rid of MockApp, and remove tricky mocking of pinia stores.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.