share weekKey extension and log splash precache failures

lib/state/app/modules/timetable/bloc/timetable_bloc.dart

@@ -1,7 +1,5 @@
 import 'dart:developer';
 
-import 'package:intl/intl.dart';
-
 import '../../../../../api/marianumconnect/queries/timetable_custom_events/custom_events_migration.dart';
 import '../../../../../api/marianumconnect/queries/timetable_get_week/timetable_get_week_response.dart';
 import '../../../../../api/mhsl/custom_timetable_event/custom_timetable_event.dart';
@@ -19,8 +17,6 @@ class TimetableBloc
           TimetableState,
           TimetableRepository
         > {
-  static final DateFormat _weekKeyFormat = DateFormat('yyyyMMdd');
-
   DateTime _lastWeekRequestStart = DateTime.fromMillisecondsSinceEpoch(0);
 
   /// Set by [retry] to force the next [gatherData] to bypass cache freshness
@@ -247,7 +243,7 @@ class TimetableBloc
   }
 
   void _writeWeekToCache(DateTime weekStart, TimetableGetWeekResponse week) {
-    final key = _weekKeyFormat.format(weekStart);
+    final key = weekStart.weekKey();
     add(
       Emit((s) {
         final updated = Map<String, TimetableGetWeekResponse>.of(s.weekCache);