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/json-stable-stringify-without-jsonify/test/cmp.js

@@ -0,0 +1,11 @@
+var test = require('tape');
+var stringify = require('../');
+
+test('custom comparison function', function (t) {
+    t.plan(1);
+    var obj = { c: 8, b: [{z:6,y:5,x:4},7], a: 3 };
+    var s = stringify(obj, function (a, b) {
+        return a.key < b.key ? 1 : -1;
+    });
+    t.equal(s, '{"c":8,"b":[{"z":6,"y":5,"x":4},7],"a":3}');
+});