feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

node_modules/supports-preserve-symlinks-flag/browser.js

@@ -0,0 +1,3 @@
+'use strict';
+
+module.exports = null;