Add Chinese Calendar

Is your feature request related to a problem? Please describe.

In China, many people use the Nongli (农历 \ Chinese Calendar) to record birthdays and anniversaries, and it is also an important calendar for daily life. But the software doesn't have a lunar calendar...（在中国，很多人使用农历记录生日和各种纪念日，同时农历也是日常生产生活的重要历法之一。然而这软件没有农历……）

Describe the solution you'd like

I want this software to display the Nongli, add Nongli events and repeat period.（我希望这软件能显示农历，添加农历日程和重复周期。）

Describe alternatives you've considered

Before the software did not have the Nongli, I could only use the email or the calendar that came with my mobile phone, such as 139 Mail, a mobile phone made in China. Different manufacturers have different Settings for the Nongli in ics, which are incompatible.（在这软件没有农历之前，我只能使用邮箱或手机自带的日程，如：139邮箱、中国产的手机。不同厂商对 ics 中的农历有不同的设置，互不兼容。）

Additional context

Add Nongli events to 139 Mail（在 139邮箱 中添加农历事件）

The correct spelling of the 农历 in English is "Chinese Calendar" or "Nongli".（农历在英语中的正确写法是“Chinese calendar”或“Nongli”。）

"Lunar Calenar" is an inaccurate translation. The 农历 is a combined calendar of Yin and Yang, and you need to calculate multiple celestial bodies to get the result.（“Lunar Calenar”是个不准确的翻译，农历是阴阳合历，需要计算多个天体才能得到结果。）

The 农历 issued by the Purple Mountain Observatory shall prevail.（以紫金山天文台发布的农历为准。）

The English content is automatically translated. Please refer to the Chinese content.（英语内容为自动翻译结果，请以中文内容为准。）

No files, tests, or entry points are named in the issue. Start by surveying the calendar app's existing event, recurrence, display, and ICS handling, then identify how Chinese Calendar dates could be represented and calculated. Done would require displaying Nongli dates, creating Nongli events, supporting their repeat periods, and confirming the calendar data matches the stated standard.