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
     }

src/app/states/skirmish_states/structures/stone.rs

@@ -4,6 +4,7 @@ use ratatui::style::Color;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Stone {
     durability: u16,
+    max_durability: u16,
     stress: u8,
 }
 
@@ -11,6 +12,7 @@ impl Stone {
     pub fn new() -> Self {
         Self {
             durability: 1000,
+            max_durability: 1000,
             stress: 0,
         }
     }
@@ -33,6 +35,10 @@ impl Structure for Stone {
         self.durability
     }
 
+    fn get_max_durability(&self) -> u16 {
+        self.max_durability
+    }
+
     fn get_stress(&self) -> u8 {
         self.stress
     }

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()
     }

src/app/states/skirmish_states/structures/structures_trait.rs

@@ -6,5 +6,6 @@ pub trait Structure {
     fn get_level(&self) -> char;
     fn get_stress(&self) -> u8;
     fn get_durability(&self) -> u16;
+    fn get_max_durability(&self) -> u16;
     fn get_name(&self) -> &'static str;
 }

src/app/states/skirmish_states/structures/tunnel.rs

@@ -4,6 +4,7 @@ use ratatui::style::Color;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Tunnel {
     durability: u16,
+    max_durability: u16,
     stress: u8,
     roof_support: bool,
     rail: bool,
@@ -14,6 +15,7 @@ impl Tunnel {
     pub fn new() -> Self {
         Self {
             durability: 500,
+            max_durability: 500,
             stress: 25,
             roof_support: false,
             rail: false,
@@ -39,6 +41,10 @@ impl Structure for Tunnel {
         self.durability
     }
 
+    fn get_max_durability(&self) -> u16 {
+        self.max_durability
+    }
+
     fn get_stress(&self) -> u8 {
         self.stress
     }