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/fast-json-stable-stringify/index.d.ts

@@ -0,0 +1,4 @@
+declare module 'fast-json-stable-stringify' {
+  function stringify(obj: any): string;
+  export = stringify;
+}