Add marking mode; rename resize and zoom methods

Introduce a marking mode that lets users toggle cells with Space and
clear
them with Backspace. BoardState now stores `marking_cells` and a list of
`marked_cells` and provides methods to start, clear, and update marks.
CellWidget tracks a `marked` flag and renders marked cells in
LightMagenta.

Keybindings are updated: Space now reads “Marking cells”, Backspace
reads
“Cancel marking”, and the original Enter binding is commented out.

The former `resize_change` and `zoom_change` functions are renamed to
`change_resize` and `change_zoom`; all call sites are updated
accordingly.

src/app/app.rs

@@ -60,7 +60,7 @@ impl App {
                     let Some(state) = self.states_mut() else {
                         panic!("State issue")
                     };
-                    state.skirmish.board.resize_change(&window_area);
+                    state.skirmish.board.change_resize(&window_area);
                 }
             }
         }