feat(htaccess): Add custom error pages for 404, 403, 500

Redirect 404, 403, and 500 HTTP errors to /404.php to provide a consistent and user-friendly error experience instead of server default pages.

public/.htaccess

@@ -34,8 +34,11 @@ DirectoryIndex index.php
 </Files>
 
 # ============================================
-# Rewrite: PHP 文件直接访问，无需额外规则
+# 自定义错误页面
 # ============================================
+ErrorDocument 404 /404.php
+ErrorDocument 403 /404.php
+ErrorDocument 500 /404.php
 
 # 禁止浏览目录
 Options -Indexes