feat: initialize JavaFX login window

src/main/java/com/university/chat/Client/UI/ChatApp.java

@@ -0,0 +1,28 @@
+package com.university.chat.Client.UI;
+
+
+import javafx.application.Application;
+import javafx.fxml.FXMLLoader;
+import javafx.scene.Scene;
+import javafx.stage.Stage;
+import javafx.stage.StageStyle;
+
+import java.io.IOException;
+
+public class ChatApp extends Application {
+
+    @Override
+    public void start(Stage stage) throws IOException {
+        FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("login.fxml"));
+        Scene scene = new Scene(fxmlLoader.load());
+        scene.getStylesheets().add(
+                getClass().getResource("/chat_style.css").toExternalForm()
+        );
+
+        stage.setTitle("Login");
+        stage.initStyle(StageStyle.UNDECORATED);
+        stage.setScene(scene);
+        stage.show();
+    }
+
+}