Adds a `mode()` method to the `Panel` enum to map panels to their
corresponding `AppMode`. Simplifies `TogglePanelAndFocus` in `cmd.rs`
to use this method instead of a manual match block.
Also adds regression tests in `app.rs` to verify that viewport
scrolling now correctly handles small terminal heights.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)
7c7fbd3289