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.rs

@@ -1,5 +1,9 @@
 use crate::app::states::skirmish_states::BoardState;
 use clap::ValueEnum;
+use ratatui::{
+    style::{Color, Stylize},
+    text::Span,
+};
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct SkirmishState {
@@ -20,3 +24,26 @@ pub enum Players {
     Player,
     Enemy,
 }
+
+impl Players {
+    pub fn get_text(&self) -> &'static str {
+        match self {
+            Self::Player => "Player",
+            Self::Enemy => "Enemy",
+        }
+    }
+
+    pub fn get_color(&self) -> Color {
+        match self {
+            Self::Player => Color::LightBlue,
+            Self::Enemy => Color::LightRed,
+        }
+    }
+
+    pub fn get_span(&self) -> Span<'static> {
+        match self {
+            Self::Player => self.get_text().fg(self.get_color()),
+            Self::Enemy => self.get_text().fg(self.get_color()),
+        }
+    }
+}