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/mz/child_process.js

@@ -0,0 +1,8 @@
+
+require('thenify-all').withCallback(
+  require('child_process'),
+  exports, [
+    'exec',
+    'execFile',
+  ]
+)