Add toggle_marking and simplify marking logic

Introduce a public `BoardState::toggle_marking` that flips the
marking mode and starts or clears marking accordingly. Refactor
`set_marked_cell` to accept tuple coordinates and clean up its
logic. Update `change_focused_cell` to work with the new signature
and use the toggle helper. Adjust cell display to show coordinates
when a cell is marked.

src/app/widgets/cell.rs

@@ -81,7 +81,7 @@ impl CellWidget {
     }
 
     fn display_coords(&self) -> Span<'_> {
-        if self.selected || self.tag != CellTag::Stone {
+        if self.selected || self.tag != CellTag::Stone || self.marked {
             format!("{}{}", self.col_to_letters(), self.row).green()
         } else {
             "".to_span()