Add bounded focus and padding to skirmish UI

- Initialize FocusedCell with map dimensions and start near the map
  centre.
- Store max rows/cols in FocusedCell and clamp moves to these bounds.
- Scroll actions now adjust the corresponding offset and also move the
  focused cell, keeping navigation consistent.
- Zoom actions modify the vertical offset to keep the view centered
  after
  changing zoom levels.
- Cell widget now uses a Padding widget with dynamic top padding based
  on
  the area height.

src/app/states/skirmish.rs

@@ -54,27 +54,34 @@ impl Offset {
 pub struct FocusedCell {
     pub row: usize,
     pub col: usize,
+    pub max_row: usize,
+    pub max_col: usize,
 }
 
 impl FocusedCell {
-    pub fn new(row: usize, col: usize) -> Self {
-        Self { row, col }
+    pub fn new(row: usize, col: usize, max_row: usize, max_col: usize) -> Self {
+        Self {
+            row,
+            col,
+            max_row,
+            max_col,
+        }
     }
 
     pub fn move_up(&mut self) {
-        self.row = self.row.saturating_sub(1);
+        self.row = self.row.saturating_sub(1).max(0);
     }
 
     pub fn move_down(&mut self) {
-        self.row = self.row.saturating_add(1);
+        self.row = self.row.saturating_add(1).min(self.max_row - 1);
     }
 
     pub fn move_left(&mut self) {
-        self.col = self.col.saturating_sub(1);
+        self.col = self.col.saturating_sub(1).max(0);
     }
 
     pub fn move_right(&mut self) {
-        self.col = self.col.saturating_add(1);
+        self.col = self.col.saturating_add(1).min(self.max_col - 1);
     }
 }