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/@babel/compat-data/corejs2-built-ins.js

@@ -0,0 +1,2 @@
+// Todo (Babel 8): remove this file as Babel 8 drop support of core-js 2
+module.exports = require("./data/corejs2-built-ins.json");