fix state disposal of sidebar on mobile layouts

lib/controllers/selected_space_controller.dart

@@ -0,0 +1,12 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+
+class SelectedSpaceController extends Notifier<int> {
+  @override
+  int build() => 0;
+
+  void set(int value) => state = value;
+
+  static final provider = NotifierProvider<SelectedSpaceController, int>(
+    SelectedSpaceController.new,
+  );
+}