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.

tests/board.rs

@@ -26,6 +26,10 @@ fn map_has_opposing_bases_ore_and_two_miners() {
       board.get_ref_cell(5, ore_col).get_structure(),
       Structures::Ore(_)
     ));
+    assert_eq!(
+      board.get_ref_cell(5, ore_col).get_structure().get_owner(),
+      Players::Unclaimed
+    );
   }
   assert_eq!(
     board