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.Server/Pages/ProspectsVerify.razor

@@ -4,9 +4,13 @@
 @inherits FsBase
 
 <PageTitle>Freischalten - @AppSettings.Terms.Title</PageTitle>
-
 <h2>Freischalten</h2>
 
+@if (AppSettings.TestMode)
+{
+    <div class="alert alert-danger"><strong>TESTMODUS!</strong> Änderungen werden wieder gelöscht.</div>
+}
+
 @{
     var filterList = ProspectList.ApplyFilter(Filter);
 }