share weekKey extension and log splash precache failures

lib/extensions/date_time.dart

@@ -64,6 +64,9 @@ extension DateTimeFormatting on DateTime {
 
   String formatRelative() => Jiffy.parseFromDateTime(this).fromNow();
 
+  /// Compact `yyyyMMdd` key, e.g. to identify a week-start in timetable caches.
+  String weekKey() => Jiffy.parseFromDateTime(this).format(pattern: 'yyyyMMdd');
+
   String timeRangeTo(DateTime end) => '${formatHm()} - ${end.formatHm()}';
 
   String formatDateRelativeShort({DateTime? now}) {