Refactor Units enum location and chain CellWidget setters

Move Units enum from the units module into the skirmish state module,
add a dedicated Players enum, and remove the now‑unused unit module.
Update all imports accordingly.
Adjust CellWidget's set_zoom_level and set_structure to return &mut
Self,
enabling method chaining.

src/app/states/skirmish.rs

@@ -1,4 +1,4 @@
-use crate::app::{buildings::BaseBuilding, states::skirmish_states::BoardState};
+use crate::app::{buildings::BaseBuilding, states::skirmish_states::BoardState, units::MinerUnit};
 use clap::ValueEnum;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -21,6 +21,12 @@ pub enum ZoomLevel {
     ZoomedOut,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Players {
+    Player,
+    Enemy,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum CellStructure {
     Base(BaseBuilding),
@@ -29,7 +35,6 @@ pub enum CellStructure {
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum Players {
-    Player,
-    Enemy,
+pub enum Units {
+    Miner(MinerUnit),
 }