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/ajv/.tonic_example.js

@@ -0,0 +1,20 @@
+var Ajv = require('ajv');
+var ajv = new Ajv({allErrors: true});
+
+var schema = {
+  "properties": {
+    "foo": { "type": "string" },
+    "bar": { "type": "number", "maximum": 3 }
+  }
+};
+
+var validate = ajv.compile(schema);
+
+test({"foo": "abc", "bar": 2});
+test({"foo": 2, "bar": 4});
+
+function test(data) {
+  var valid = validate(data);
+  if (valid) console.log('Valid!');
+  else console.log('Invalid: ' + ajv.errorsText(validate.errors));
+}