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/states/skirmish.rs

@@ -1,12 +1,39 @@
 use clap::ValueEnum;
-use ratatui::widgets::ScrollbarState;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct Offset {
+    value: usize,
+    max: usize,
+}
+
+impl Offset {
+    pub fn new() -> Self {
+        Self { value: 0, max: 0 }
+    }
+
+    pub fn get_value(&self) -> usize {
+        self.value
+    }
+
+    pub fn set_max(&mut self, max: usize) {
+        self.max = max;
+    }
+
+    pub fn next(&mut self) {
+        self.value = self.value.saturating_add(1).min(self.max);
+    }
+
+    pub fn prev(&mut self) {
+        self.value = self.value.saturating_sub(1).max(0)
+    }
+}
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SkirmishState {
     pub id: usize,
     pub name: &'static str,
-    pub vertical_scrollbar: ScrollbarState,
-    pub horizontal_scrollbar: ScrollbarState,
+    pub vertical_offset: Offset,
+    pub horizontal_offset: Offset,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]