Allow to inspect content of generated files in TUI

Currently we show project structure on-the-fly. But it would be better to make `Project structure` widget smarter, like, scrollable list that allows to open file and browse it's content so people can see what will they get.

Start by locating the TUI code that renders the `Project structure` widget and review how generated files are represented and navigated. Determine how scrolling, file selection, and content browsing fit the existing interaction model. Done means users can select a generated file from the widget, browse its content, and return to the project structure view.

Written by the indexing model from the issue text.