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/base.rs

@@ -4,6 +4,7 @@ use ratatui::style::Color;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct BaseBuilding {
     durability: u16,
+    max_durability: u16,
     stress: u8,
     owner: Players,
     level: u8,
@@ -13,6 +14,7 @@ impl BaseBuilding {
     pub fn new(owner: Players) -> Self {
         Self {
             durability: 1500,
+            max_durability: 1500,
             stress: 0,
             owner,
             level: b'1',
@@ -44,6 +46,10 @@ impl Structure for BaseBuilding {
         self.durability
     }
 
+    fn get_max_durability(&self) -> u16 {
+        self.max_durability
+    }
+
     fn get_stress(&self) -> u8 {
         self.stress
     }