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/array-union/index.d.ts

@@ -0,0 +1,25 @@
+/**
+Create an array of unique values, in order, from the input arrays.
+
+@example
+```
+import arrayUnion = require('array-union');
+
+arrayUnion([1, 1, 2, 3], [2, 3]);
+//=> [1, 2, 3]
+
+arrayUnion(['foo', 'foo', 'bar']);
+//=> ['foo', 'bar']
+
+arrayUnion(['🐱', '🦄', '🐻'], ['🦄', '🌈']);
+//=> ['🐱', '🦄', '🐻', '🌈']
+
+arrayUnion(['🐱', '🦄'], ['🐻', '🦄'], ['🐶', '🌈', '🌈']);
+//=> ['🐱', '🦄', '🐻', '🐶', '🌈']
+```
+*/
+declare function arrayUnion<ArgumentsType extends readonly unknown[]>(
+	...arguments: readonly ArgumentsType[]
+): ArgumentsType;
+
+export = arrayUnion;