Add a hint in the desktop client if the server is not up and running

[sabaimran]

There should be a basic health check in the client that tries to verify that the Khoj server that the user has configured is indeed running. It should help localize the problem for the user better.

See relevant discussion in Discord.

[Sanket-Arekar]

Hey @sabaimran I would like to work on this issue. Can you Please assign me this issue?