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/settings.rs

@@ -6,7 +6,7 @@ pub struct SettingsState {
   pub game_mode: GameMode,
   pub perk_deck: PerkDecks,
   pub starting_wood: u16,
-  pub starting_iron: u16,
+  pub starting_metal: u16,
   pub supply_limit: u8,
   pub xp_modifier: f32,
   pub skill_points_limit: u16,