feat: Add standalone message TITLE and HTML/Markdown content support

[cuteribs-1]

Describe the solution you'd like

• Send Markdown/HTML to API. (Server)
• Display message title in message list (App)
• Display HTML (or translated from Markdown) content in a detail page (App)

Describe alternatives you've considered
Like what ServerChan did for Wechat.

[long2ice]

Good to have this.

[MyLovePoppet]

附议，现在使用的是pushdeer，可以展示markdown格式的消息，但是pushdeer很久没更新了，在ios18上兼容性已经无法得到保证，希望作者能够增加对markdown或者HTML格式消息的展示。这个应用扩展性和可玩性还是比pushdeer要高不少的😋

[xiaoxinpro]

是的 如果能支持markdown格式就太棒了！