feat: 增强用户端配置管理功能

主要修改:
1. 在SystemConfigController中完善用户端配置获取和批量更新接口的实现。
2. 优化SystemConfigService，增强配置值验证逻辑，确保配置的准确性和有效性。

技术细节:
- 新增的功能提升了用户端配置的管理灵活性，支持更高效的批量操作。

src/main/java/com/gameplatform/server/service/admin/AnnouncementService.java

@@ -0,0 +1,94 @@
+package com.gameplatform.server.service.admin;
+
+import com.gameplatform.server.mapper.admin.AnnouncementMapper;
+import com.gameplatform.server.model.entity.admin.Announcement;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.List;
+
+@Service
+@Transactional
+public class AnnouncementService {
+    
+    @Autowired
+    private AnnouncementMapper announcementMapper;
+    
+    /**
+     * 创建公告
+     */
+    public boolean createAnnouncement(Announcement announcement) {
+        return announcementMapper.insert(announcement) > 0;
+    }
+    
+    /**
+     * 根据ID获取公告
+     */
+    public Announcement getAnnouncementById(Long id) {
+        return announcementMapper.findById(id);
+    }
+    
+    /**
+     * 更新公告
+     */
+    public boolean updateAnnouncement(Announcement announcement) {
+        return announcementMapper.update(announcement) > 0;
+    }
+    
+    /**
+     * 删除公告
+     */
+    public boolean deleteAnnouncement(Long id) {
+        return announcementMapper.deleteById(id) > 0;
+    }
+    
+    /**
+     * 获取所有公告（分页）
+     */
+    public List<Announcement> getAllAnnouncements(int size, int offset) {
+        return announcementMapper.findAll(size, offset);
+    }
+    
+    /**
+     * 获取公告总数
+     */
+    public long getAnnouncementCount() {
+        return announcementMapper.countAll();
+    }
+    
+    /**
+     * 根据启用状态获取公告（分页）
+     */
+    public List<Announcement> getAnnouncementsByEnabled(Boolean enabled, int size, int offset) {
+        return announcementMapper.findByEnabled(enabled, size, offset);
+    }
+    
+    /**
+     * 根据启用状态获取公告数量
+     */
+    public long getAnnouncementCountByEnabled(Boolean enabled) {
+        return announcementMapper.countByEnabled(enabled);
+    }
+    
+    /**
+     * 更新公告启用状态
+     */
+    public boolean updateAnnouncementEnabled(Long id, Boolean enabled) {
+        return announcementMapper.updateEnabled(id, enabled) > 0;
+    }
+    
+    /**
+     * 获取所有启用的公告（用于前端显示）
+     */
+    public List<Announcement> getEnabledAnnouncements() {
+        return announcementMapper.findByEnabled(true, 10, 0); // 最多返回10条启用的公告
+    }
+    
+    /**
+     * 检查公告是否存在
+     */
+    public boolean announcementExists(Long id) {
+        return getAnnouncementById(id) != null;
+    }
+}