Add ore level support and unify text rendering

Introduce a numeric `level` field for `BaseBuilding` and `Ore`, update
their
constructors and `get_level` implementations, and increase the default
ore
amount to the new maximum. Extend the `Structure` and `Unit` traits with
`base_text` and `text` methods, add `is_unit` to `Units`, and adjust the
`Structures` enum to delegate text rendering. Refactor `SidePanelWidget`
to
use the unified trait methods, removing duplicated rendering code.
Update
`BoardState` to create ore structures with an initial level of 1.

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

@@ -15,7 +15,7 @@ impl BaseBuilding {
             durability: 1500,
             stress: 0,
             owner,
-            level: b'1',
+            level: 1,
         }
     }
 }
@@ -34,7 +34,7 @@ impl Structure for BaseBuilding {
     }
 
     fn get_level(&self) -> char {
-        self.level as char
+        (self.level + b'0') as char
     }
 
     fn get_durability(&self) -> u16 {