Add scrolling actions and board widget

Introduce directional and scroll actions, update keybindings, add
scrollbar
state to SkirmishState, and render the board with a new BoardWidget.
Adjust CLI
defaults and layout heights accordingly.

src/app/states/skirmish.rs

@@ -1,9 +1,12 @@
 use clap::ValueEnum;
+use ratatui::widgets::ScrollbarState;
 
 #[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,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]