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/resolve/SECURITY.md

@@ -0,0 +1,11 @@
+# Security
+
+Please file a private vulnerability via GitHub, email [@ljharb](https://github.com/ljharb), or see https://tidelift.com/security if you have a potential security vulnerability to report.
+
+## Incident Response
+
+See our [Incident Response Process](.github/INCIDENT_RESPONSE_PROCESS.md).
+
+## Threat Model
+
+See [THREAT_MODEL.md](./THREAT_MODEL.md).