Add Stone and Tunnel, refactor CellStructure

Introduce new building types Stone and Tunnel with their own tag, color,
and
properties. CellStructure now holds these structs (Tunnel, Stone)
instead of
simple variants. BaseBuilding is simplified to return a char tag and a
color
based on owner. BoardState creation and CellWidget rendering are updated
to
use the new building structs and their methods.

src/app/states/skirmish_states/units/miner.rs

@@ -3,15 +3,14 @@ use crate::app::states::Players;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct MinerUnit {
     owner: Players,
-    tag: &'static str,
 }
 
 impl MinerUnit {
     pub fn new(owner: Players) -> Self {
-        Self { owner, tag: "M" }
+        Self { owner }
     }
 
-    pub fn get_tag(self) -> &'static str {
-        self.tag
+    pub fn get_tag(self) -> char {
+        'M'
     }
 }