Move buildings and units under skirmish_states

Reorganize the codebase by moving the buildings and units modules
into the skirmish_states namespace, updating imports accordingly.

src/app/states/skirmish_states/board.rs

@@ -1,11 +1,9 @@
 use crate::app::{
-    buildings::BaseBuilding,
     helpers::{CellSizes, cell_size_helper, cells_area_helper},
     states::{
         CellStructure, FocusedCell, Offset, Players, Units, ZoomLevel,
-        skirmish_states::MoveFocusedCell,
+        skirmish_states::{MoveFocusedCell, buildings::BaseBuilding, units::MinerUnit},
     },
-    units::MinerUnit,
     widgets::CellWidget,
 };
 use ratatui::layout::Rect;