Refactor skirmish tasks and update dependencies

- Disable automatic board.advance_turn and replace it with explicit unit
  movement handling.
- Add Tunnel structure creation during digging tasks.
- Extend DiggingTask with pop_front and current_target helpers.
- Rename task getters to get_ref_tasks and add mutable get_mut_tasks.
- Update Unit trait and Units enum to expose these new methods.
- Remove unused EMPTY_TASKS lazy static.
- Bump flake.lock revisions, hashes, and timestamps for several
  dependencies.

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

@@ -16,6 +16,15 @@ impl DiggingTask {
             tick_increase,
         }
     }
+
+    pub fn pop_front(&mut self) {
+        self.path.pop_front();
+        self.progress = 0.0;
+    }
+
+    pub fn current_target(&self) -> Option<(usize, usize)> {
+        self.path.front().copied()
+    }
 }
 
 impl Task for DiggingTask {