feat(auth): 优化图形验证码生成逻辑，新增自定义颜色支持

(cherry picked from commit af900df10d6e9df4b9812332f126c2e96474722e)

pigx-auth/src/main/java/com/pig4cloud/pigx/auth/endpoint/ImageCodeEndpoint.java

@@ -18,6 +18,8 @@ import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
 
+import java.awt.Color;
+
 /**
  * 验证码相关的接口
  *
@@ -39,7 +41,7 @@ public class ImageCodeEndpoint {
     @SneakyThrows
     @GetMapping("/image")
     public void image(String randomStr, HttpServletResponse response) {
-        ArithmeticCaptcha captcha = new ArithmeticCaptcha(DEFAULT_IMAGE_WIDTH, DEFAULT_IMAGE_HEIGHT);
+        ArithmeticCaptcha captcha = createCaptcha();
 
         String result = captcha.text();
 
@@ -53,6 +55,15 @@ public class ImageCodeEndpoint {
         captcha.out(response.getOutputStream());
     }
 
+    static ArithmeticCaptcha createCaptcha() {
+        return new ArithmeticCaptcha(DEFAULT_IMAGE_WIDTH, DEFAULT_IMAGE_HEIGHT) {
+            @Override
+            protected Color color() {
+                return Color.BLACK;
+            }
+        };
+    }
+
     /**
      * 行为 创建滑块图形验证码
      */