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/browserslist/error.d.ts

@@ -0,0 +1,7 @@
+declare class BrowserslistError extends Error {
+  constructor(message: any)
+  name: 'BrowserslistError'
+  browserslist: true
+}
+
+export = BrowserslistError