fix no room selected after leave / init

lib/controllers/selected_space.dart

@@ -0,0 +1,25 @@
+import "package:collection/collection.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/key.dart";
+import "package:nexus/controllers/spaces.dart";
+import "package:nexus/models/space.dart";
+
+class SelectedSpaceController extends Notifier<Space?> {
+  @override
+  Space? build() {
+    final spaces = ref.watch(SpacesController.provider).value;
+    if (spaces == null) return null;
+
+    final spaceId = ref.watch(
+      KeyController.provider(KeyController.spaceKey)
+          .select((async) => async.value),
+    );
+
+    return spaces.firstWhereOrNull((space) => space.id == spaceId) ??
+        spaces.firstOrNull;
+  }
+
+  static final provider = NotifierProvider<SelectedSpaceController, Space?>(
+    SelectedSpaceController.new,
+  );
+}