implemented background prefetching for files root, added 24-hour caching for root directory listing, and enabled cache renewal for manual refreshes

lib/api/marianumcloud/webdav/queries/list_files/list_files_cache.dart

@@ -16,8 +16,9 @@ class ListFilesCache extends SimpleCache<ListFilesResponse> {
     super.onNetworkData,
     super.onError,
     required String path,
+    super.renew = false,
   }) : super(
-         cacheTime: RequestCache.cacheNothing,
+         cacheTime: _cacheTimeFor(path),
          loader: () => ListFiles(ListFilesParams(path)).run(),
          fromJson: ListFilesResponse.fromJson,
          onUpdate: onUpdate,
@@ -25,6 +26,44 @@ class ListFilesCache extends SimpleCache<ListFilesResponse> {
     start(_documentId(path));
   }
 
+  /// The Nextcloud root listing is significantly slower than subfolders on
+  /// our instance and frequently returns HTTP 500. Since its content rarely
+  /// changes, the root payload is cached for a full day so app-resume and
+  /// connectivity-change auto-refetch triggers do not re-hit the slow root
+  /// endpoint within the same day. To avoid a long wait on the very first
+  /// open of the Files page, `prefetchRootListing` (called from `main`)
+  /// kicks off an async warm-up fetch in the background while the user is
+  /// still on the launch screen / other modules. Subfolders keep the
+  /// previous "always refetch on visit" TTL because their content changes
+  /// more often. Explicit user refreshes (rename, delete, copy/move,
+  /// upload) bypass the TTL via the inherited [renew] flag or via
+  /// [invalidate].
+  static int _cacheTimeFor(String path) {
+    final stripped = path.replaceAll('/', '').trim();
+    return stripped.isEmpty
+        ? RequestCache.cacheDay
+        : RequestCache.cacheNothing;
+  }
+
+  /// Triggers a root-listing fetch in the background if no cached payload
+  /// exists yet. Intended to be called once after login from `main` so the
+  /// (slow) root listing is already populated by the time the user
+  /// navigates to the Files module.
+  ///
+  /// No-ops when a cached root payload is already present in localstore —
+  /// the regular TTL handling in [RequestCache] takes over from there.
+  static Future<void> prefetchRootListing() async {
+    const rootPath = '';
+    final cached = await Localstore.instance
+        .collection(RequestCache.collection)
+        .doc(_documentId(rootPath))
+        .get();
+    if (cached != null) return;
+    // Drive the same code path as a regular fetch so the result lands in
+    // the cache; we don't care about the in-memory callback here.
+    ListFilesCache(path: rootPath, onUpdate: (_) {});
+  }
+
   static String _documentId(String path) {
     final cacheName = md5
         .convert(utf8.encode('MarianumMobile-$path'))