refactor(biji-houdaun): 重构用户认证方法并添加安全工具类

- 将 MarkdownController 中的 isUserAuthenticated 方法移至 SecurityUtil 工具类- 在 SecurityUtil 中添加以下新方法：
  - getCurrentUsername - getCurrentUserDetails
  - hasRole
  - hasPermission - isAdmin
- 更新 MarkdownController 中的相关调用，使用 SecurityUtil 的静态方法

biji-houdaun/src/main/java/com/test/bijihoudaun/controller/MarkdownController.java

@@ -5,14 +5,12 @@ import com.test.bijihoudaun.common.response.R;
 import com.test.bijihoudaun.entity.MarkdownFile;
 import com.test.bijihoudaun.entity.MarkdownFileVO;
 import com.test.bijihoudaun.service.MarkdownFileService;
+import com.test.bijihoudaun.util.SecurityUtil;
 import io.swagger.v3.oas.annotations.Operation;
 import io.swagger.v3.oas.annotations.Parameter;
 import io.swagger.v3.oas.annotations.Parameters;
 import io.swagger.v3.oas.annotations.tags.Tag;
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.security.core.Authentication;
-import org.springframework.security.core.context.SecurityContextHolder;
-import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.web.bind.annotation.*;
 
 import java.util.Date;
@@ -40,7 +38,7 @@ public class MarkdownController {
     @GetMapping("/{id}")
     public R<String> getMarkdownContent(@PathVariable Long id) {
         // 获取当前认证状态
-        boolean isAuthenticated = isUserAuthenticated();
+        boolean isAuthenticated = SecurityUtil.isUserAuthenticated();
         
         MarkdownFile file = markdownFileService.getMarkdownById(id, isAuthenticated);
         if (ObjectUtil.isNotNull(file)) {
@@ -113,19 +111,4 @@ public class MarkdownController {
         return R.success(files);
     }
 
-    /**
-     * 检查用户是否已认证
-     * @return 是否已认证
-     */
-    private boolean isUserAuthenticated() {
-        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
-        if (authentication == null || !authentication.isAuthenticated()) {
-            return false;
-        }
-        Object principal = authentication.getPrincipal();
-        if (principal instanceof UserDetails) {
-            return true;
-        }
-        return false;
-    }
 }