Add "All" prospects view and refine filtering logic

Introduced a new "All" prospects page for admins to view and manage all prospects. Removed unused "Users" list and related logic from code. Updated filters to support the new "All" state and adjusted navigation to include the new page.

FoodsharingSiegen.Server/Pages/ProspectsAll.razor

@@ -0,0 +1,21 @@
+@page "/all"
+
+@using FoodsharingSiegen.Shared.Helper
+@inherits FsBase
+
+<PageTitle>Freischalten - @AppSettings.Terms.Title</PageTitle>
+
+<h2>Freischalten</h2>
+
+@{
+    var filterList = ProspectList.ApplyFilter(Filter);
+}
+
+<hr/>
+<ProspectFilterControl @bind-Filter="Filter" StateFilter="ProspectStateFilter.All"></ProspectFilterControl>
+<hr />
+<ProspectGrid
+    Prospects="filterList"
+    OnDataChanged="@LoadProspects"
+    StateFilter="ProspectStateFilter.All">
+</ProspectGrid>