Refactor & robust terminal restore

[panicbit]

This PR refactors most of the smaller functions.
The most significant refactoring changes include:

• using let-else to reduce nesting
• using .context() / .with_context() to add context to anyhow's errors
• liberal use of variables to prevent long lines

Additionally, the main app loop got extracted from main into the run_app function.
The reason for this is to ensure that the terminal is always getting restored properly,
even if the app returns an error.

I'm very tempted to also split up some of the larger functions and move some stuff into new modules, but last time I tried that I ended up with nasty merge conflicts 🥲.

[bitmagier]

Nice facelift. Thanks.