Use VecDeque for tasks and marked cells

Replace `Vec` with `VecDeque` in `BoardState` marked cells,
`DiggingTask` path,
and `MinerUnit` tasks to enable efficient push/pop operations. Remove
the
`Copy` trait from several structs and enums, adjust derives accordingly,
and
use `as_ref` in `Option<Units>` implementations to avoid consuming the
option.
Update cell rendering to clone widgets and adjust
`CellWidget::get_option_unit`
to return a cloned `Option<Units>`. Add necessary `VecDeque` imports.

src/app/widgets/board.rs

@@ -38,7 +38,7 @@ impl Widget for BoardWidget<'_> {
                     .cells
                     .get(row_idx + self.state.vertical_offset.get_value())
                     .and_then(|rows| rows.get(col_idx + self.state.horizontal_offset.get_value()))
-                    .map(|cell| cell.render(*cell_area, buf));
+                    .map(|cell| cell.clone().render(*cell_area, buf));
             }
         }
     }