Standardize sprite_index property and add scale_x/scale_y to UISprite

closes #81, closes #82

- Issue #81: Standardized property name to sprite_index across UISprite and UIEntity
  - Added sprite_index as the primary property name
  - Kept sprite_number as a deprecated alias for backward compatibility
  - Updated repr() methods to use sprite_index
  - Updated animation system to recognize both names

- Issue #82: Added scale_x and scale_y properties to UISprite
  - Enables non-uniform scaling of sprites
  - scale property still works for uniform scaling
  - Both properties work with the animation system

All existing code using sprite_number continues to work due to backward compatibility.

src/Animation.cpp

@@ -90,8 +90,8 @@ void Animation::startEntity(UIEntity* target) {
             }
         }
         else if constexpr (std::is_same_v<T, int>) {
-            // For entities, we might need to handle sprite_number differently
-            if (targetProperty == "sprite_number") {
+            // For entities, we might need to handle sprite_index differently
+            if (targetProperty == "sprite_index" || targetProperty == "sprite_number") {
                 startValue = target->sprite.getSpriteIndex();
             }
         }