fix(backend): replace blocking functions with async functions

[ravenclaw900]

No description provided.

[ravenclaw900]

Alright, a small rise in binary size, however now everything is actually being done correctly. The size change is slightly offset by the fact that we can now switch to the single-threaded tokio runtime. Also, various file browser improvements:

• Not being able to create/delete files or directories in the root directory
• Files not being zipped, just downloaded as they are
• Directories not using a compressed zip (would tar work on Windows instead?)
• Being able to stop a directory read/download that's taking a while and switch to reading/downloading a different directory