fix: 添加删除帖子的 mock handler

- 支持 DELETE /api/posts/:postId 请求
- 从内存存储中正确删除评论
- 修复 mock 模式下删除评论失败的问题

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Co-Authored-By: Claude <noreply@anthropic.com>

src/services/eventService.js

@@ -210,7 +210,8 @@ export const eventService = {
                     ...post,
                     author: post.user ? {
                         id: post.user.id,
-                        username: post.user.username,
+                        // 与导航区保持一致：优先显示昵称
+                        username: post.user.nickname || post.user.username,
                         avatar: post.user.avatar_url || post.user.avatar  // 兼容 avatar_url 和 avatar
                     } : {
                         id: 'anonymous',