better loading indicators for timetables, talk and files

lib/state/app/modules/files/dataProvider/files_data_provider.dart

@@ -1,5 +1,3 @@
-import 'dart:async';
-
 import 'package:nextcloud/nextcloud.dart';
 
 import '../../../../../api/marianumcloud/webdav/queries/listFiles/listFilesCache.dart';
@@ -7,15 +5,30 @@ import '../../../../../api/marianumcloud/webdav/queries/listFiles/listFilesRespo
 import '../../../../../api/marianumcloud/webdav/webdavApi.dart';
 
 class FilesDataProvider {
-  Future<ListFilesResponse> listFiles(String path) {
-    final completer = Completer<ListFilesResponse>();
-    ListFilesCache(
+  /// Lists files at [path]. Cached payload is delivered via [onCacheData] as
+  /// soon as it is read from disk, so callers can render stale data while the
+  /// network call is still pending. The Future itself resolves once both the
+  /// cache lookup and the network attempt have settled, throwing if no payload
+  /// could be obtained at all.
+  Future<ListFilesResponse> listFiles(
+    String path, {
+    void Function(ListFilesResponse)? onCacheData,
+    void Function(Object)? onError,
+  }) async {
+    ListFilesResponse? latest;
+    Object? capturedError;
+    final cache = ListFilesCache(
       path: path,
-      onUpdate: (data) {
-        if (!completer.isCompleted) completer.complete(data);
+      onUpdate: (data) => latest = data,
+      onCacheData: onCacheData,
+      onError: (e) {
+        capturedError = e;
+        onError?.call(e);
       },
     );
-    return completer.future;
+    await cache.ready;
+    if (latest != null) return latest!;
+    throw capturedError ?? Exception('No data and no error from listFiles');
   }
 
   Future<void> createFolder(String fullPath) async {