feat(PostIt): unify moderator administration and restore Android navigation

Group users and usage in the moderator administration page; retain personal profile management and show personal usage. Allow moderators to read the corresponding API lists while preserving administrator-only mutations. Restore navigation pages after Android activity recreation. Update tests, architecture documentation and CHANGELOG for 1.0.8-rc27.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/PostIt/PostIt.Tests/ActivityNavigationTests.cs

@@ -0,0 +1,60 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using PostIt.ViewModels;
+using PostIt.Views;
+
+namespace PostIt.Tests;
+
+public class ActivityNavigationTests
+{
+    [AvaloniaFact]
+    public async Task Activity_recreation_restores_fresh_pages_and_back_navigation()
+    {
+        var app = Assert.IsType<App>(Application.Current);
+        var previousView = app.View;
+        var original = new MainView();
+        var home = new HomePage();
+        var draft = new Settings { SelectedLanguage = "fr" };
+        var editor = new ContentPage { DataContext = draft };
+        await original.NavRoot.PushAsync(home, null);
+        await original.NavRoot.PushAsync(editor, null);
+        app.AttachMainWindow(original);
+        var window = new Window();
+        try
+        {
+            var restored = app.CreateActivityView(new Settings());
+            window.Content = restored;
+            window.Show();
+            Dispatcher.UIThread.RunJobs();
+
+            Assert.Equal(2, restored.NavRoot.NavigationStack.Count);
+            Assert.IsType<HomePage>(restored.NavRoot.NavigationStack[0]);
+            Assert.NotSame(home, restored.NavRoot.NavigationStack[0]);
+            Assert.NotSame(editor, restored.NavRoot.NavigationStack[1]);
+            Assert.Same(draft, restored.NavRoot.NavigationStack[1].DataContext);
+            Assert.Equal("fr", draft.SelectedLanguage);
+
+            window.Content = null;
+            window.Content = restored;
+            Dispatcher.UIThread.RunJobs();
+            Assert.Equal(2, restored.NavRoot.NavigationStack.Count);
+
+            // Another activity recreation must retain the already-restored stack.
+            window.Content = null;
+            var recreatedAgain = app.CreateActivityView(new Settings());
+            window.Content = recreatedAgain;
+            Dispatcher.UIThread.RunJobs();
+            Assert.Equal(2, recreatedAgain.NavRoot.NavigationStack.Count);
+            Assert.Same(draft, recreatedAgain.NavRoot.NavigationStack[1].DataContext);
+            await recreatedAgain.NavRoot.PopAsync(null);
+            Assert.IsType<HomePage>(Assert.Single(recreatedAgain.NavRoot.NavigationStack));
+        }
+        finally
+        {
+            window.Close();
+            app.AttachMainWindow(previousView ?? new MainView());
+        }
+    }
+}