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/@eslint/eslintrc/universal.js

@@ -0,0 +1,9 @@
+// Jest (and probably some other runtimes with custom implementations of
+// `require`) doesn't support `exports` in `package.json`, so this file is here
+// to help them load this module. Note that it is also `.js` and not `.cjs` for
+// the same reason - `cjs` files requires to be loaded with an extension, but
+// since Jest doesn't respect `module` outside of ESM mode it still works in
+// this case (and the `require` in _this_ file does specify the extension).
+
+// eslint-disable-next-line no-undef
+module.exports = require("./dist/eslintrc-universal.cjs");