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/state.rs

@@ -32,7 +32,12 @@ impl GameStates {
                 map_height: args.map_height as usize,
                 board_cells: Vec::new(),
                 zoom_level: args.zoom_level,
-                focused_cell: FocusedCell::new(0, 0),
+                focused_cell: FocusedCell::new(
+                    (args.map_height / 2) as usize - 1,
+                    2,
+                    args.map_height as usize,
+                    args.map_width as usize,
+                ),
             },
             perk_decks: PerkDecksState {
                 id: 2,