lan-im: LAN Instant Messaging and File Sharing

`lan-im` is a DeepSeek Harness (DSH) plugin designed to meet local area network (LAN) instant messaging and file sharing needs for multi-node collaboration. It uses mDNS for node discovery and supports an account system, offline message delivery in private chats, WebSocket-based real-time push, and desktop notifications. Each channel has an independent file space with file indexing and caching. It is easy to install and can be launched via the sidebar or a shortcut, making it suitable for use within a trusted LAN. Note: Messages are transmitted in plaintext without end-to-end encryption, and offline private chat messages are limited to 200.