Separate birthday events into a dedicated ICS source

[wen-templari]

Background

The current calendar plugin fetches a single iCal source (https://ical.nbtca.space/) which contains all events including birthdays. Birthday events should be separated into a dedicated ICS file to allow independent management.

Requirements

• Extract birthday events out of the main public ICS into a separate ICS source
• The bot should continue to send QQ group notifications for events from both sources (main calendar + birthday calendar)
• No existing notification behavior should be broken

Proposed Solution

Add multi-ICS URL support to the Calendar plugin:

1. Add ICalUrls string[] field to PluginConfig (replacing the hardcoded single URL)
2. Update RemoteSync to fetch all configured URLs in parallel and merge the resulting calendars
3. Pass Config.ICalUrls from PluginMain into RemoteSync

Example config:

"Calendar": {
  "ICalUrls": [
    "https://ical.nbtca.space/",
    "https://ical.nbtca.space/birthday"
  ]
}

All downstream logic (timer, filtering, QQ messaging) remains unchanged since they operate on a single merged Ical.Net.Calendar object.

[wen-templari]

@LazuliKao Do you think this requirement is valid? If you think we can continue with this solution, I will open a pull request.