implemented short relative date formatting for chat and added unit tests

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',
+      );
+    });
+  });
 }