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/fast-deep-equal/es6/react.d.ts

@@ -0,0 +1,2 @@
+declare const equal: (a: any, b: any) => boolean;
+export = equal;