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/digging.rs

@@ -1,5 +1,7 @@
 use std::collections::VecDeque;
 
+use crate::app::states::skirmish_states::tasks::Task;
+
 #[derive(Debug, Clone, PartialEq)]
 pub struct DiggingTask {
     path: VecDeque<(usize, usize)>,
@@ -10,3 +12,21 @@ impl DiggingTask {
         Self { path }
     }
 }
+
+impl Task for DiggingTask {
+    fn get_icon(&self) -> char {
+        '⛏'
+    }
+
+    fn get_path(&self) -> &VecDeque<(usize, usize)> {
+        &self.path
+    }
+
+    fn get_path_front_coords(&self) -> &(usize, usize) {
+        self.path.front().map_or(&(0, 0), |c| c)
+    }
+
+    fn get_path_back_coords(&self) -> &(usize, usize) {
+        self.path.back().map_or(&(0, 0), |c| c)
+    }
+}