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/keybindings/skirmish.rs

@@ -7,10 +7,10 @@ use ratatui::crossterm::event::KeyEvent;
 pub fn skirmish_keybindings(app: &mut App, key_event: &KeyEvent) {
     if let Some(action) = event_to_action(&key_event) {
         match action {
-            Action::ScrollUp => app.states.skirmish.vertical_scrollbar.prev(),
-            Action::ScrollDown => app.states.skirmish.vertical_scrollbar.next(),
-            Action::ScrollLeft => app.states.skirmish.horizontal_scrollbar.prev(),
-            Action::ScrollRight => app.states.skirmish.horizontal_scrollbar.next(),
+            Action::ScrollUp => app.states.skirmish.vertical_offset.prev(),
+            Action::ScrollDown => app.states.skirmish.vertical_offset.next(),
+            Action::ScrollLeft => app.states.skirmish.horizontal_offset.prev(),
+            Action::ScrollRight => app.states.skirmish.horizontal_offset.next(),
             Action::Quit => app.exit = true,
             Action::Quit2 => app.exit = true,
             Action::Esc => app.view = View::MainMenu,