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/inherits/inherits.js

@@ -0,0 +1,9 @@
+try {
+  var util = require('util');
+  /* istanbul ignore next */
+  if (typeof util.inherits !== 'function') throw '';
+  module.exports = util.inherits;
+} catch (e) {
+  /* istanbul ignore next */
+  module.exports = require('./inherits_browser.js');
+}