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/cell.rs

@@ -11,7 +11,7 @@ use ratatui::{
     widgets::{Block, BorderType, Borders, Paragraph, Widget},
 };
 
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, PartialEq)]
 pub struct CellWidget {
     row: usize,
     col: usize,
@@ -57,7 +57,7 @@ impl CellWidget {
     }
 
     pub fn get_option_unit(&self) -> Option<Units> {
-        self.unit
+        self.unit.clone()
     }
 
     pub fn set_structure(&mut self, structure: Structures) -> &mut Self {