Supporting tree structure instead of flat list in navigation pane

Hi!

Just wanted to pretext, that installed revdiff 2 or 3 days ago, really love the project! It was taking me a long time to find a terminal diff viewer that fit the agentic workflow I liked and revdiff is hitting that sweet spot. Anyway heres an issue i'd like to flag with a ready PR if the maintainers allow. I was going to set up a PR initially but read the CONTRIBUTING.md and realized I should put up an issue first in case it doesnt fit the product direction.

Problem

current files are shown as a flat list in the navigation pane. Coming from an IDE perspective it's a bit hard to read and understand. I opted for an option to the user if they want to show a tree instead of just a flat list! Again, fully configurable and set by the user. Clicking Enter or space on the folder will collapse them while keeping the same behavior of clicking on the file becomes reviewed (same as before).

https://github.com/user-attachments/assets/040ce37e-19d3-454c-8ffc-1907adb2a822

Read CONTRIBUTING.md first, then locate the navigation pane entry point and its handling for Enter, space, and file selection. Confirm how a user-configurable tree view should preserve file-review behavior while allowing folders to collapse. Done means the pane supports both flat and tree views with the requested folder and file interactions.