fixed possible lockups on app start

lib/utils/exponential_backoff.dart

@@ -0,0 +1,11 @@
+/// Delay before retry number [attempt] (1-based): [base] doubled per attempt,
+/// capped at [max].
+Duration exponentialBackoff(
+  int attempt, {
+  Duration base = const Duration(milliseconds: 500),
+  Duration max = const Duration(seconds: 5),
+}) {
+  final exponent = (attempt - 1).clamp(0, 30);
+  final millis = base.inMilliseconds * (1 << exponent);
+  return millis >= max.inMilliseconds ? max : Duration(milliseconds: millis);
+}