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/@ungap/structured-clone/cjs/json.js

@@ -0,0 +1,24 @@
+'use strict';
+/*! (c) Andrea Giammarchi - ISC */
+
+const {deserialize} = require('./deserialize.js');
+const {serialize} = require('./serialize.js');
+
+const {parse: $parse, stringify: $stringify} = JSON;
+const options = {json: true, lossy: true};
+
+/**
+ * Revive a previously stringified structured clone.
+ * @param {string} str previously stringified data as string.
+ * @returns {any} whatever was previously stringified as clone.
+ */
+const parse = str => deserialize($parse(str));
+exports.parse = parse;
+
+/**
+ * Represent a structured clone value as string.
+ * @param {any} any some clone-able value to stringify.
+ * @returns {string} the value stringified.
+ */
+const stringify = any => $stringify(serialize(any, options));
+exports.stringify = stringify;