feat: optimize email filtering with frontend-only search (#787)

* feat: optimize email filtering with frontend-only search

- Remove backend keyword parameter from mail APIs (breaking change)
- Implement frontend filtering on current page (20-100 items)
- Add message_id database index for UPDATE performance
- Support desktop and mobile responsive layouts
- Update API documentation and CHANGELOG

BREAKING CHANGE: /admin/mails and /user_api/mails no longer accept keyword parameter

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* fix: restore Mail ID query input in Index.vue

- Keep showMailIdQuery UI input for querying specific mail by ID
- Triggered when URL contains mail_id parameter

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -3,6 +3,8 @@
 
 ## v1.2.0(main)
 
+- feat: |邮件过滤| 移除后端 keyword 参数，改为前端过滤当前页邮件，优化查询性能
+- feat: |数据库| 为 `message_id` 字段添加索引，优化邮件更新操作性能，需执行 `db/2025-12-15-message-id-index.sql` 更新数据库
 - feat: |Admin| 维护页面增加自定义 SQL 清理功能，支持定时任务执行自定义清理语句
 
 ## v1.1.0