add sticker and emote support

lib/controllers/thumbnail_controller.dart

@@ -0,0 +1,24 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:matrix/matrix.dart";
+import "package:nexus/controllers/client_controller.dart";
+import "package:nexus/models/image_data.dart";
+
+class ThumbnailController extends AsyncNotifier<String?> {
+  ThumbnailController(this.data);
+  final ImageData data;
+
+  @override
+  Future<String?> build({String? from}) async {
+    final client = await ref.watch(ClientController.provider.future);
+    final uri = await Uri.tryParse(data.uri)?.getDownloadUri(
+      client,
+    ); // TODO: Should use thumb when c10y fixes animated thumbs
+
+    return uri.toString();
+  }
+
+  static final provider = AsyncNotifierProvider.family
+      .autoDispose<ThumbnailController, String?, ImageData>(
+        ThumbnailController.new,
+      );
+}