Replace scrollbar state with simple offset struct

Introduce Offset type to track scroll positions and replace
ScrollbarState fields in SkirmishState
Update keybindings, App::draw, Widget impl, and BoardWidget::new
to use mutable references and the new Offset struct.
Adjust imports accordingly.

src/app/app.rs

@@ -52,7 +52,7 @@ impl App {
         Ok(())
     }
 
-    fn draw(&self, frame: &mut Frame<'_>) {
+    fn draw(&mut self, frame: &mut Frame<'_>) {
         frame.render_widget(self, frame.area());
     }