feat: support modifying existing group announcements via _send_group_notice

[Copilot]

Every announcement update created a new entry, with no way to edit an existing one. This adds an optional notice_id field to _send_group_notice — when provided, the request includes fid in the QQ web API call, causing the server to update the existing announcement in-place.

Changes

• webapi.ts — setGroupNotice accepts an optional noticeId parameter; when non-empty, passes it as fid to add_qun_notice
• SendGroupNotice.ts — adds optional notice_id to the payload schema; forwards it to setGroupNotice

Usage

Obtain the notice_id from _get_group_notice, then pass it to update:

{
  "action": "_send_group_notice",
  "params": {
    "group_id": "123456",
    "content": "Updated content",
    "notice_id": "<fid from _get_group_notice>"
  }
}

Omitting notice_id preserves the existing behaviour (creates a new announcement).