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/emoji-regex/text.d.ts

@@ -0,0 +1,5 @@
+declare module 'emoji-regex/text' {
+  function emojiRegex(): RegExp;
+
+  export = emojiRegex;
+}