test: add tests for presence system

[Aaromalpm]

Context

src/presence.js was added in #14 but has no tests. The existing test suite lives in test/test.js.

What to test

• trackPresence(socket, userId) — emits presence:change with status: 'online' when first socket for a user connects
• trackPresence — does not re-emit presence:change if the same user connects from a second tab (already online)
• Disconnect — emits presence:change with status: 'offline' only when the last socket for a user disconnects
• Disconnect — does not emit offline if the user still has another socket open
• setPresence(userId, 'away') — emits presence:change with status: 'away'
• setPresence() — throws if userId is not tracked
• setPresence() — throws if status is invalid
• getPresence(userId) — returns { status: 'offline', lastSeen: null } for unknown user
• getRoomPresence(roomId) — returns a map of all participants' presence

Setup tip

Use socket.io-client (already a devDependency) to spin up a real server and connect test clients, similar to the existing auth tests.

[shamalag]

Wanted to start with this issue but when I ran the existing test 3 got failed out of 33