Add Task trait and improve miner task display

Define a full `Task` trait with icon and path accessors. Implement the
trait for
`DiggingTask` and delegate it through the `Tasks` enum. Introduce helper
functions in `MinerUnit` to convert coordinates to spreadsheet‑style
strings
and use them in a revamped `get_tasks_formatted`. Update the unit
rendering
logic to show the tasks queue only when tasks are present.

src/app/states/skirmish_states/tasks/task_trait.rs

@@ -1 +1,11 @@
-pub trait Task {}
+use std::collections::VecDeque;
+
+pub trait Task {
+    fn get_icon(&self) -> char;
+
+    fn get_path(&self) -> &VecDeque<(usize, usize)>;
+
+    fn get_path_front_coords(&self) -> &(usize, usize);
+
+    fn get_path_back_coords(&self) -> &(usize, usize);
+}