added support for simultan downloads in files and talk, support for background downloads, enhanced loading in files with retry

lib/api/retry.dart

@@ -0,0 +1,38 @@
+import 'package:nextcloud/nextcloud.dart';
+
+/// True for errors worth a short automatic retry: HTTP 5xx thrown by the
+/// nextcloud/dynamite clients. Our instance intermittently answers the slow
+/// root PROPFIND with a quick empty 500 via nginx that succeeds on retry.
+/// Timeouts are deliberately not transient — the root listing already has a
+/// multi-minute ceiling, so repeating it would multiply the wait.
+bool isTransientServerError(Object error) =>
+    error is DynamiteApiException &&
+    error.statusCode >= 500 &&
+    error.statusCode <= 599;
+
+Duration _defaultRetryDelay(int retry) =>
+    retry == 1 ? const Duration(seconds: 1) : const Duration(seconds: 3);
+
+/// Runs [attempt] up to [maxAttempts] times, waiting [delayFor] between
+/// tries. Errors that don't match [shouldRetry] and the final failure are
+/// rethrown unchanged. [onRetry] fires before each wait with the 1-based
+/// number of the upcoming attempt, e.g. `(2, 3)` — used to surface retry
+/// progress in the UI. [delayFor] receives the 1-based index of the retry
+/// being scheduled and is injectable so tests run instantly.
+Future<T> retryOnTransientError<T>(
+  Future<T> Function() attempt, {
+  int maxAttempts = 3,
+  Duration Function(int retry) delayFor = _defaultRetryDelay,
+  bool Function(Object error) shouldRetry = isTransientServerError,
+  void Function(int nextAttempt, int maxAttempts)? onRetry,
+}) async {
+  for (var attemptNo = 1; ; attemptNo++) {
+    try {
+      return await attempt();
+    } catch (e) {
+      if (attemptNo >= maxAttempts || !shouldRetry(e)) rethrow;
+      onRetry?.call(attemptNo + 1, maxAttempts);
+      await Future<void>.delayed(delayFor(attemptNo));
+    }
+  }
+}