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-json-stable-stringify/example/str.js

@@ -0,0 +1,3 @@
+var stringify = require('../');
+var obj = { c: 6, b: [4,5], a: 3 };
+console.log(stringify(obj));