Group files by status (unstaged/staged/untracked) in tree view

## Is your feature request related to a problem? Please describe.

In tree view, it's difficult to quickly distinguish between untracked files, unstaged changes, and staged changes. Files are organized by directory structure without any grouping by git status, making it harder to see the working tree state at a glance.

## Describe the solution you'd like

Add visual section headers in the Files panel (tree view) to separate files by status:

--- Untracked files ---
?? foo.md
--- Unstaged changes ---
▼ pkg/gui
M command_log_panel.go
--- Staged changes ---
A boo.sh

This would provide clear visual grouping similar to neogit, while maintaining the directory tree structure within each section.

## Describe alternatives you've considered
- Color coding - exists but less obvious than section headers

## Additional context
I'm Happy to take this one as a first contribution to the project :)