Allow configuring the flexible split threshold for main panel layout

### Is your feature request related to a problem? Please describe.
I use a tiling window manager. When lazygit is the only window open, my terminal is 193 columns wide, which is just below the hardcoded 200 column threshold that determines whether the main panel splits horizontally or vertically.

### Describe the solution you'd like
A way to configure the horizontal split threshold.

The current code is:
https://github.com/jesseduffield/lazygit/blob/722239abd1edb756f4d6e65b8b7b0ba05c10011c/pkg/gui/controllers/helpers/window_arrangement_helper.go#L389-L390

Maybe something like this:
```
gui:
mainPanelSplitMode: flexible
mainPanelSplitHorizontalThreshold: 150 # default 200
```

### Describe alternatives you've considered
I tried setting `mainPanelSplitMode: horizontal`, and it works the way i want (horizontal split) when lazygit is the only window open, but if I open another window the terminal halves in width, and I dont want it stay a horizontal split at this smaller width.

Another alternative is for me to use `ctrl + -` to decrease my terminal font size, but I like my current size and don't want to have to decrease it just to meet the hardcoded value.

### Additional context
At 193 columns wide, flexible mode splits vertically even though I'd prefer horizontal so that I can actually see more than a few lines of the diff:

Start at pkg/gui/controllers/helpers/window_arrangement_helper.go around lines 389-390, where the flexible layout uses the hardcoded threshold, and trace how GUI settings are defined and read. Add a configurable horizontal threshold with a default of 200, then verify that flexible mode changes its split at the configured width while retaining the existing behavior by default.

Written by the indexing model from the issue text.