UX: Add one-key shortcuts for direct panel navigation

[lecheel]

git-igitt is a great project, and the existing arrow navigation works really well for linear browsing. However, jumping from panel 1 to 4 currently takes multiple keystrokes. This PR adds complementary one-key shortcuts (with numeric labels in the UI) so users can jump directly to any panel while keeping the arrow flow intact. Tested locally—would appreciate your review!

[peso]

First of all, thank you for your contribution. It is really appreciated. Below I have tried to argue for why I think it is not the best way to achieve your goal. Also there are some extra requests from my side to make your code fit better into the project. I hope you have time to consider these suggestions.

Your patch
This works, but you have "0" as the branches window, which breaks the left-right flow because that key is to the far right on the keyboard. Also, there is no key "4" for the diff panel. It would also be nice to have it mentioned on the help page (F1). Of course "0" could be left out, as we already have "B" to toggle the branch list (see F1 help page). It is not the same thing, but close.

Motivation
Why do you want to navigate fast to the files panel? I believe it is to see the diff.
I think this illustrates a workflow "locate commit, look at diff" that is worth optimizing the UI for.

Suggested change A
We could make it a zero-keystroke operation by changing the 3 commit-files-diff panels to one panel showing a combined message and diff, which looks like the output of git show
We could add key "F" to toggle between file display (3-panels) and show display (1-panel).

Suggested change B
If this is too much work, I suggest you change your code to let "F" jump between log (current key 1) and files (key 3). This would go under heading "Layout/panels" on the help page.
The show panel mentioned above will then become a feature request.