Add Miner stats and conditional audio flag

- Extend MinerUnit with hp, digging ability, and power, and provide
  getters for these fields.
- Update the Unit trait and Units enum to expose hp, max hp,
  can_dig, and digging_power, and enhance base_text to display them.
- Remove unnecessary `Eq` derives from several structs and enums.
- Introduce a `--disable-audio` CLI flag and conditionally spawn the
  audio thread based on its value.

src/app/states/skirmish.rs

@@ -1,6 +1,6 @@
 use crate::app::states::skirmish_states::BoardState;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq)]
 pub struct SkirmishState {
     pub id: usize,
     pub name: &'static str,