Refactor buildings/units, move CellTag & Players

Introduce BaseBuilding struct. Rename Building and Unit to enum types
(Buildings, Units). Move CellTag and Players enums to the skirmish state
module and update imports and re‑exports accordingly. Add placeholder
miner module.

src/app/buildings/building.rs

@@ -1,2 +1,2 @@
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct Building {}
+pub enum Buildings {}