Drop Singleton pattern and implement Dependency Injection

Instead of using a Singleton pattern, ensure data consistency and prevent duplication of `allAnimals` by passing the same `AnimalService` instance between controllers.
Additionally, ensure data is not loaded twice by setting a flag that must be checked before calling `loadFromDb()`.
This maintains the benefits previously established by the Singleton while reducing complexity and improving testability.

src/main/java/com/crud/gsjavafx/utils/DependencyFactory.java

@@ -0,0 +1,10 @@
+package com.crud.gsjavafx.utils;
+
+import com.crud.gsjavafx.models.AnimalService;
+
+public class DependencyFactory {
+    public static AnimalService createAnimalService() {
+        RescueAnimalDAO dao = new RescueAnimalDAO();
+        return new AnimalService(dao);
+    }
+}