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/units/units_enum.rs

@@ -42,4 +42,8 @@ impl Unit for Option<Units> {
     fn get_owner(&self) -> Players {
         self.map_or(Players::Unclaimed, |u| u.get_owner())
     }
+
+    fn is_unit(&self) -> bool {
+        self.map_or(false, |u| u.is_unit())
+    }
 }