[idea] Add z keybinding to center the cursor line in copy mode

[irwinarruda]

idea / problem

I am coming from tmux, where I use prefix + [ copy mode heavily. One tmux habit I miss is pressing z to center the current cursor line in the viewport.

That behavior is useful when reading scrollback: I can navigate to the line I care about, press z, and continue reading with that line centered instead of stuck near the top or bottom of the pane.

requested change

Add a z keybinding in copy mode that centers the current cursor line vertically in the visible viewport.

Expected behavior:

• In normal copy/navigation mode, pressing z centers the current cursor line.
• In visual selection mode, pressing z keeps the active selection intact and centers the active cursor line.
• Near the top or bottom of scrollback, the viewport clamps normally.

I've tried solving the issue already locally. The core idea is:

   'z' => self.copy_mode_scroll_middle(terminal_runtimes),

With an implementation that computes the current absolute row, moves the viewport so that row lands around inner_rect.height / 2, clamps at scrollback boundaries, then syncs the copy-mode selection again.

why you want this

This would make Herdr match even more tmux's copy mode behaviour.

I know only a little Rust today, but with AI help I was able to prototype the small change locally and get a clearer sense of how Herdr’s copy-mode state works. If I'm approved to contribute, I would for sure learn Rust properly so I can contribute more over time.