Leave room via WebSocket

JoinRoom is handled in the session's recv task, but LeaveRoom is never matched -- the variant exists in ChatMessage and in RouterMessage but there's no match arm for it in session.rs

The JoinRoom arm is handled at line 211 of session.rs, but LeaveRoom falls through to the Ok(_) => {} catch-all. Wiring it up is a few lines -- send RouterMessage::LeaveRoom the same way JoinRoom sends RouterMessage::JoinRoom

Start in session.rs at the JoinRoom match arm around line 211 and compare it with the LeaveRoom variant in ChatMessage and RouterMessage. Wire LeaveRoom through the session's recv task so it sends RouterMessage::LeaveRoom rather than falling through the Ok(_) catch-all; done when leaving a room is routed like joining one.

Written by the indexing model from the issue text.