postit: change MainPage base class from NavigationPage to ContentPage

HomePage.OnLoginClick does Navigation.PushAsync(new MainPage
{ ... }). NavigationPage.PushAsync only accepts a Page (or
Page subclass), not a MultiPage. MainPage was declared as

  public partial class MainPage : NavigationPage

in MainPage.axaml.cs and the root of MainPage.axaml was

  <NavigationPage ...>

which means 'new MainPage()' produced a MultiPage, not a
Page. PushAsync against a MultiPage argument does not route
through the standard Page push path; the visible result is
that the login succeeds, LoginSucceeded fires, but the UI
stays on LoginPage — the user is left looking at the
post-login state without any navigation.

The XAML content of MainPage (a StackPanel with the post
CRUD UI, a ListBox, a TextEditor) does not need the
multi-page container semantics — it's a single screen.
Switch both the code-behind base class and the XAML root
element to ContentPage so that MainPage is what
PushAsync expects.

src/PostIt/PostIt/Views/MainPage.axaml

@@ -1,4 +1,4 @@
-<NavigationPage xmlns="https://github.com/avaloniaui"
+<ContentPage xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
@@ -57,4 +57,4 @@
             </Border>
         
     </StackPanel>
-</NavigationPage>
+</ContentPage>

src/PostIt/PostIt/Views/MainPage.axaml.cs

@@ -1,10 +1,9 @@
-
 using Avalonia;
 using Avalonia.Controls;
 
 namespace PostIt.Views;
 
-public partial class MainPage  : NavigationPage
+public partial class MainPage : ContentPage
 {
     public MainPage()
     {