Add zoom support and dynamic keybinding layout

Introduce ZoomIn/ZoomOut actions and a Zoom group with corresponding
keybindings.
Add a CLI option to set the initial zoom level and a ZoomLevel enum.
Expose a utility to compute the largest keybinding group size for layout
sizing.
Refactor board widget to adjust cell dimensions based on zoom level and
simplify offset calculations.
Update views to use the new layout sizing helper and integrate zoom
handling in skirmish logic.

src/app/views/main_menu.rs

@@ -1,4 +1,8 @@
-use crate::app::{App, View, keybindings::Action, widgets::KeybindingsWidget};
+use crate::app::{
+    App, View,
+    keybindings::{Action, count_largest_group},
+    widgets::KeybindingsWidget,
+};
 use clap::ValueEnum;
 use ratatui::{
     buffer::Buffer,
@@ -23,7 +27,18 @@ fn format_view_string(s: String) -> String {
 }
 
 pub fn main_menu_view(app: &App, area: Rect, buf: &mut Buffer) {
-    let vertical_layout: Layout = Layout::vertical([Constraint::Fill(1), Constraint::Length(4)]);
+    let actions: Vec<Action> = vec![
+        Action::Up,
+        Action::Down,
+        Action::Space,
+        Action::Quit,
+        Action::Quit2,
+    ];
+
+    let vertical_layout: Layout = Layout::vertical([
+        Constraint::Fill(1),
+        Constraint::Length(count_largest_group(&actions) + 2),
+    ]);
 
     let [main_menu_area, keybindings_area] = vertical_layout.areas(area);
 
@@ -83,14 +98,6 @@ pub fn main_menu_view(app: &App, area: Rect, buf: &mut Buffer) {
     }
 
     {
-        let actions: Vec<Action> = vec![
-            Action::Up,
-            Action::Down,
-            Action::Space,
-            Action::Quit,
-            Action::Quit2,
-        ];
-
         KeybindingsWidget::new(actions).render(keybindings_area, buf);
     }
 }