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/path-exists/index.d.ts

@@ -0,0 +1,28 @@
+declare const pathExists: {
+	/**
+	Check if a path exists.
+
+	@returns Whether the path exists.
+
+	@example
+	```
+	// foo.ts
+	import pathExists = require('path-exists');
+
+	(async () => {
+		console.log(await pathExists('foo.ts'));
+		//=> true
+	})();
+	```
+	*/
+	(path: string): Promise<boolean>;
+
+	/**
+	Synchronously check if a path exists.
+
+	@returns Whether the path exists.
+	*/
+	sync(path: string): boolean;
+};
+
+export = pathExists;