Refactor UI into separate components

[nazar-pc]

Right now the whole UI is packed in main.rs. It was easier to start that way, but as more features are added and existing features become more complex, it is clear that the current approach doesn't scale, especially when doing things like #13.

What needs to happen is that UI needs to be sliced first into separate components one per screen (roughly) and then into further sub-components as necessary. Backend is already composed from several logical modules, we need something similar for the fronend too.

This might be a bit tricky, but it is worth doing and is a blocker for major UI work.