Adjust side panel width at runtime

**Is your feature request related to a problem? Please describe.**
I think it would improve UX if it's allowed to adjust side panel width at run time, both normal screen mode and half screen mode. Especially for half screen mode, I always think that the side panel takes up too many space, although we can focus on the main panel now, it's would still be a good feature to allow user navigating files with adjustable panel width at run time.

**Describe the solution you'd like**
Add two new states at `GuiRepoState` storing the current adjusted side panel width per screen mode.
Add two new keybindings (to be discussed what keys would be the best) to adjust the side panel width at run time.

**Additional context**
In fact, I have twisted the codebase to fit my own usage. Here is a demonstration of proof of concept. I'm willing to contribute to this feature and make it more thorough if there's interest from the community.

https://github.com/user-attachments/assets/531aa5d9-1d1e-477a-be5d-8e6e5f680ef1