Close all connections on server shutdown and settings refresh

Node's server.close() stops accepting new connections but leaves
existing keep-alive connections open indefinitely. This caused Obsidian
to hang on plugin unload or settings refresh until clients happened to
close their connections on their own.

Calling closeAllConnections() before close() immediately terminates
all open sockets, ensuring the server shuts down promptly. Also bumps
@types/node from ^16 to ^18 to pick up the closeAllConnections type
definition (Node 18.2+, available in Obsidian 1.4.0+ via Electron 25).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>