improve dateFormat a bit, and fix some buttons

flutter/lib/state/app_settings.dart

@@ -30,6 +30,17 @@ enum AppSettingsDateFormat {
     }
   }
 
+  DateFormat dateOnlyFormat() {
+    switch (this) {
+      case AppSettingsDateFormat.ISO:
+        return DateFormat('yyyy-MM-dd');
+      case AppSettingsDateFormat.German:
+        return DateFormat('dd.MM.yyyy');
+      case AppSettingsDateFormat.US:
+        return DateFormat('MM/dd/yyyy');
+    }
+  }
+
   static AppSettingsDateFormat? parse(String? string) {
     if (string == null) return null;
     return values.firstWhere((e) => e.key == string, orElse: null);