Function handle_key 

pub fn handle_key(action: KeyCommand, app: &mut AppState) -> AppAction

Handle an action while in SquashSelect mode.

The user navigates the commit list to pick a squash target. The source commit (from source_index) will be squashed into the chosen target. Navigation is clamped so the cursor cannot move to commits later than the source — squashing into a later commit is not supported.