Add "TestMode" flag to display warnings in test environments

Introduced a new "TestMode" setting in AppSettings to toggle test mode. Updated multiple pages to display a warning message when "TestMode" is enabled. This ensures users are informed when changes may not be persistent.

FoodsharingSiegen.Contracts/Model/AppSettings.cs

@@ -3,9 +3,11 @@
     public class AppSettings
     {
         #region Public Properties
-        
+
         public TermSettings Terms { get; set; } = new();
 
+        public bool TestMode { get; set; }
+
         #endregion
     }
 }