Rename iron to metal, add mining and recruitment

- Updated CLI, README and documentation to use `--starting-metal` with
  `--starting-iron` retained as an alias.
- Introduced `Resources` struct and constants for wood delivery and mine
  production.
- Added new structures `Mine` and `MineConstruction` with cost,
  production
  and cancel logic.
- Implemented building view for constructing mines and recruitment view
  for spawning miners.
- Extended keybindings with `Building` and `Recruitment` actions and
  updated
  action availability logic.
- Modified game state, board, units and UI widgets to handle metal
  resources
  and new construction mechanics.
- Updated tests and increased total test count to 82, covering
  resources,
  construction, recruitment and context‑aware keybindings.

src/app/states/skirmish_states/units/unit_trait.rs

@@ -22,6 +22,10 @@ pub trait Unit {
 
   fn get_can_dig(&self) -> bool;
 
+  fn get_can_build(&self) -> bool {
+    false
+  }
+
   fn get_digging_power(&self) -> f32;
 
   fn get_coords(&self) -> (usize, usize);
@@ -66,6 +70,10 @@ pub trait Unit {
       ]));
     }
 
+    if self.get_can_build() {
+      lines.push(Line::from("Can build: Yes").green());
+    }
+
     if !have_any_tasks {
       lines.push(Line::from("Tasks Queue: ".gray()));
       for task in tasks {