Refactor keybindings with groups and symbols

Introduce a `Group` enum to categorize actions and add `group` and
`symbol`
fields to `KeyBinding`. Extend `Action` with `Up`, `Down`, `Space`, and
`Esc`, updating all keybinding definitions accordingly.

Update `binding_for_view` to return the new actions per view and handle
`Esc` by returning to the main menu.

Rewrite the keybindings widget as a custom `Widget` that renders grouped
paragraphs with proper layout.

Adjust the main menu layout percentages to accommodate the new widget.

src/app/app.rs

@@ -103,6 +103,7 @@ impl App {
                 if let Some(action) = event_to_action(&key_event) {
                     match action {
                         Action::Quit => self.exit = true,
+                        Action::Esc => self.view = View::MainMenu,
                         _ => (),
                     }
                 }