feat: add ElectricCar with battery system

src/main/java/org/example/model/ElectricCar.java

@@ -0,0 +1,35 @@
+package org.example.model;
+
+/*
+ * ElectricCar extends Car.
+ * It already inherits Serviceable from Car.
+ *
+ * Service logic for ElectricCar:
+ * - needsService(): return true if mileage > 8000
+ * - performService():
+ *     Print "Battery system checked"
+ *     Reset chargeLevel to 100
+ */
+
+public class ElectricCar extends Car {
+
+    private int batteryCapacity;
+    private double chargeLevel;
+
+    public ElectricCar(String brand, String model, int year,
+                       double mileage, int seats,
+                       int batteryCapacity, double chargeLevel) {
+
+        super(brand, model, year, mileage, seats);
+        this.batteryCapacity = batteryCapacity;
+        this.chargeLevel = chargeLevel;
+    }
+
+    // TODO: Override getEnergyType() → return "Electric"
+
+    // TODO: Override showDetails()
+
+    // TODO: Implement charge(double amount)
+    // Validate negative
+    // Cap at 100
+}