Add DisableStepIn flag to control StepIn-related interactions

Introduced the `DisableStepIn` flag in AppSettings to toggle StepIn-related features. Updated UI components to conditionally render StepIn interactions and filtering options based on this flag. Refactored group permissions for interactions to include Ambassadors where applicable.

FoodsharingSiegen.Contracts/Model/AppSettings.cs

@@ -4,6 +4,8 @@
     {
         #region Public Properties
 
+        public bool DisableStepIn { get; set; }
+
         public TermSettings Terms { get; set; } = new();
 
         public bool TestMode { get; set; }