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/corejs3-shipped-proposals.js

@@ -0,0 +1,2 @@
+// Todo (Babel 8): remove this file now that it is included in babel-plugin-polyfill-corejs3
+module.exports = require("./data/corejs3-shipped-proposals.json");