Refactor skirmish focus handling and CellWidget API

Introduce a MoveFocusedCell enum and a BoardState.change_focused_cell
method to
centralize focus movement logic. Update skirmish keybindings to use this
new
method and simplify board rendering by directly using stored CellWidget
state.
Make CellWidget setters chainable and expose the new enum in the module
re‑exports. Remove duplicated max_offset logic and old move_* methods.

src/app/states/skirmish_states/mod.rs

@@ -3,5 +3,5 @@ pub mod focused_cell;
 pub mod offset;
 
 pub use board::BoardState;
-pub use focused_cell::FocusedCell;
+pub use focused_cell::{FocusedCell, MoveFocusedCell};
 pub use offset::Offset;