Supporting configurable tree pane position

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

The file tree and Markdown TOC are fixed to the left side. I wanted this to be configurable so that user can put the file tree on the right side in case they wanted to. Since some users (also me) normally use their file trees on the right side for navigation in the IDE.

heres a video of how i imagined it which existed in my fork. if its good ill open a PR for it! the navigation of h and k becomes directional rather than just hardcoded to focus on a specific pane.

https://github.com/user-attachments/assets/3593ac9c-976b-40e5-a88f-29cf3f003b27

Start by tracing the TUI layout for the file tree and Markdown TOC, then follow the existing h and k pane-navigation handling. Run revdiff and compare the current fixed-left behavior with the linked video. Done means the tree position is configurable and navigation moves directionally between panes without breaking the existing layout.