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/tailwindcss/stubs/.prettierrc.json

@@ -0,0 +1,6 @@
+{
+  "printWidth": 120,
+  "semi": false,
+  "singleQuote": true,
+  "trailingComma": "es5"
+}