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();
+  }
 }

test/extensions/date_time_test.dart

@@ -58,4 +58,53 @@ void main() {
       expect(dt.timeRangeTo(end), '09:07 - 09:52');
     });
   });
+
+  group('formatDateRelativeShort', () {
+    final now = DateTime(2026, 5, 9, 14, 0); // Saturday
+
+    test('today returns "Heute"', () {
+      expect(
+        DateTime(2026, 5, 9, 8, 0).formatDateRelativeShort(now: now),
+        'Heute',
+      );
+    });
+
+    test('yesterday returns "Gestern"', () {
+      expect(
+        DateTime(2026, 5, 8, 23, 30).formatDateRelativeShort(now: now),
+        'Gestern',
+      );
+    });
+
+    test('2 to 6 days ago returns the German weekday name', () {
+      // 2026-05-07 is a Thursday
+      expect(
+        DateTime(2026, 5, 7).formatDateRelativeShort(now: now),
+        'Donnerstag',
+      );
+      // 2026-05-03 is a Sunday (6 days before Saturday 9th)
+      expect(
+        DateTime(2026, 5, 3).formatDateRelativeShort(now: now),
+        'Sonntag',
+      );
+    });
+
+    test('7 days or more ago falls back to dd.MM.yyyy', () {
+      expect(
+        DateTime(2026, 5, 2).formatDateRelativeShort(now: now),
+        '02.05.2026',
+      );
+      expect(
+        DateTime(2026, 1, 1).formatDateRelativeShort(now: now),
+        '01.01.2026',
+      );
+    });
+
+    test('future dates fall back to dd.MM.yyyy', () {
+      expect(
+        DateTime(2026, 5, 10).formatDateRelativeShort(now: now),
+        '10.05.2026',
+      );
+    });
+  });
 }