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/board.rs

@@ -81,9 +81,9 @@ impl BoardState {
                 } else if enemy_base {
                     Structures::Base(BaseBuilding::new(Players::Enemy))
                 } else if player_ore {
-                    Structures::Ore(Ore::new(Players::Player))
+                    Structures::Ore(Ore::new(Players::Player, 1))
                 } else if enemy_ore {
-                    Structures::Ore(Ore::new(Players::Enemy))
+                    Structures::Ore(Ore::new(Players::Enemy, 1))
                 } else {
                     Structures::Stone(Stone::new())
                 };