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/postcss-js/index.mjs

@@ -0,0 +1,8 @@
+import index from './index.js'
+
+export default index
+
+export const objectify = index.objectify
+export const parse = index.parse
+export const async = index.async
+export const sync = index.sync