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/view.rs

@@ -13,7 +13,7 @@ pub enum View {
     SkillsConfig,
 }
 
-impl Widget for &App {
+impl Widget for &mut App {
     fn render(self, area: Rect, buf: &mut Buffer)
     where
         Self: Sized,