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/structures/ore.rs

@@ -32,6 +32,12 @@ impl Ore {
     5000
   }
 
+  pub fn extract(&mut self, amount: u16) -> u16 {
+    let extracted: u16 = amount.min(self.ore_amount);
+    self.ore_amount -= extracted;
+    extracted
+  }
+
   pub fn set_owner(&mut self, new_owner: Players) -> &mut Self {
     self.owner = new_owner;
     self