Add max durability and enhance side panel UI

- Introduce `max_durability` fields to BaseBuilding, Stone, and Tunnel
  structures.
- Extend `Structure` trait with `get_max_durability` and implement it in
  all structures.
- Update `Structures` enum to forward `get_max_durability`.
- Add color utilities and `get_span` method to `Players` enum for styled
  text.
- Refactor `CellWidget` method names (`get_option_unit`,
  `set_structure`).
- Revise side panel widget to accept references to `Structures` and
  `Option<Units>`, render colored blocks and detailed stats.
- Simplify skirmish view to use `BoardState` directly and adapt side
  panel rendering.

src/app/widgets/cell.rs

@@ -56,12 +56,12 @@ impl CellWidget {
         self.structure
     }
 
-    pub fn get_unit_option(&self) -> Option<Units> {
+    pub fn get_option_unit(&self) -> Option<Units> {
         self.unit
     }
 
-    pub fn set_structure(&mut self, sctructure: Structures) -> &mut Self {
-        self.structure = sctructure;
+    pub fn set_structure(&mut self, structure: Structures) -> &mut Self {
+        self.structure = structure;
         self
     }