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/states/skirmish_states/structures/structures_enum.rs

@@ -40,6 +40,10 @@ impl Structure for Structures {
         self.structure().get_durability()
     }
 
+    fn get_max_durability(&self) -> u16 {
+        self.structure().get_max_durability()
+    }
+
     fn get_stress(&self) -> u8 {
         self.structure().get_stress()
     }