implemented short relative date formatting for chat and added unit tests

lib/api/marianumcloud/talk/chat/get_chat_response.dart

@@ -66,7 +66,7 @@ class GetChatResponseObject {
 
   static GetChatResponseObject getDateDummy(int timestamp) {
     var elementDate = DateTime.fromMillisecondsSinceEpoch(timestamp * 1000);
-    return getTextDummy(elementDate.formatDate());
+    return getTextDummy(elementDate.formatDateRelativeShort());
   }
 
   static GetChatResponseObject getTextDummy(String text) =>

lib/extensions/date_time.dart

@@ -46,4 +46,18 @@ extension DateTimeFormatting on DateTime {
   String formatRelative() => Jiffy.parseFromDateTime(this).fromNow();
 
   String timeRangeTo(DateTime end) => '${formatHm()} - ${end.formatHm()}';
+
+  String formatDateRelativeShort({DateTime? now}) {
+    final reference = now ?? DateTime.now();
+    final today = DateTime(reference.year, reference.month, reference.day);
+    final self = DateTime(year, month, day);
+    final diff = today.difference(self).inDays;
+
+    if (diff == 0) return 'Heute';
+    if (diff == 1) return 'Gestern';
+    if (diff > 1 && diff <= 6) {
+      return Jiffy.parseFromDateTime(this).format(pattern: 'EEEE');
+    }
+    return formatDate();
+  }
 }