fix: 修复GUI模块编译错误，添加缺失的JavaFX import

gui/src/main/java/com/smartlibrary/gui/GUIApplication.java

@@ -10,6 +10,9 @@ import com.smartlibrary.service.BookService;
 import com.smartlibrary.service.UserService;
 import com.smartlibrary.uml.PlantUMLService;
 import javafx.stage.FileChooser;
+import javafx.geometry.Orientation;
+import javafx.scene.image.Image;
+import javafx.scene.image.ImageView;
 import java.io.ByteArrayInputStream;
 import java.io.File;
 import java.util.List;
@@ -36,8 +39,18 @@ import java.time.LocalDate;
  */
 public class GUIApplication extends Application {
     
+    // 样式常量
+    private static final String STYLE_CARD = "-fx-background-color: white; -fx-border-radius: 5; -fx-border-color: #e0e0e0;";
+    private static final String STYLE_WHITE_BG = "-fx-background-color: white;";
+    private static final String COLOR_GREEN = "#4caf50";
+    private static final String COLOR_ORANGE = "#ff9800";
+    private static final String COLOR_BLUE = "#2196f3";
+    private static final String UML_TYPE_CLASS = "class";
+    private static final String MSG_EMPTY_PUML = "请先输入或生成PlantUML代码！";
+    private static final String MSG_INVALID_PUML = "PlantUML代码无效，请检查语法！";
+    private static final String ROLE_READER = "READER";
+    
     private BookService bookService;
-    private AIService aiService;
     private PlantUMLService plantUMLService;
     private UserService userService;
     private Stage primaryStage;
@@ -57,7 +70,7 @@ public class GUIApplication extends Application {
     public void start(Stage primaryStage) {
         this.primaryStage = primaryStage;
         bookService = new BookService();
-        aiService = new SmartAIService(bookService);
+        AIService aiService = new SmartAIService(bookService);
         plantUMLService = new PlantUMLService();
         userService = UserService.getInstance();
         
@@ -93,9 +106,6 @@ public class GUIApplication extends Application {
         
         // 创建场景
         Scene scene = new Scene(root, 1200, 800);
-        // 暂时注释掉CSS加载，避免空CSS文件导致问题
-        // scene.getStylesheets().add(getClass().getResource("/styles.css").toExternalForm());
-        
         primaryStage.setScene(scene);
         primaryStage.setMinWidth(1000);
         primaryStage.setMinHeight(700);