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/@nodelib/fs.scandir/out/utils/fs.d.ts

@@ -0,0 +1,2 @@
+import type { Dirent, Stats } from '../types';
+export declare function createDirentFromStats(name: string, stats: Stats): Dirent;