Track window area and refactor keybinding APIs

- Track terminal size in `App::window_area` and update on resize
- Accept action slices in `count_largest_group` and
  `KeybindingsWidget::new`
- Add ACTIONS slices and layout helpers for default, main_menu, skirmish
- Add `main_menu_option_helper` for formatting menu strings and export
  it

src/app/helpers/main_menu_option.rs

@@ -0,0 +1,13 @@
+pub fn main_menu_option_helper(s: String) -> String {
+    if let Some(pos) = s
+        .char_indices()
+        .filter(|(_, c)| c.is_ascii_uppercase())
+        .nth(1)
+        .map(|(i, _)| i)
+    {
+        let (first, second) = s.split_at(pos);
+        format!("{first} {second}")
+    } else {
+        s
+    }
+}