Show space hierarchies

Fixes #83

lib/controllers/space_hierarchy.dart

@@ -0,0 +1,34 @@
+import "package:hooks_riverpod/hooks_riverpod.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/models/space_hierarchy.dart";
+
+class SpaceHierarchyController(final String spaceId)
+    extends AsyncNotifier<SpaceHierarchy> {
+  @override
+  Future<SpaceHierarchy> build() => ref
+      .read(ClientController.provider.notifier)
+      .getSpaceHierarchy(.new(roomId: spaceId, maxDepth: 1));
+
+  Future<void> loadMore() async {
+    final current = await future;
+    if (current.nextBatch == null) return;
+
+    final next = await ref
+        .read(ClientController.provider.notifier)
+        .getSpaceHierarchy(
+          .new(roomId: spaceId, maxDepth: 1, from: current.nextBatch),
+        );
+
+    state = AsyncData(
+      current.copyWith(
+        rooms: current.rooms.addAll(next.rooms),
+        nextBatch: next.nextBatch,
+      ),
+    );
+  }
+
+  static final provider = AsyncNotifierProvider.family
+      .autoDispose<SpaceHierarchyController, SpaceHierarchy, String>(
+        SpaceHierarchyController.new,
+      );
+}