PostIt.Android: drive the PKCE flow through Chrome Custom Tabs

The earlier commit removed the client_secret and wired
MainActivity.OnNewIntent to AndroidOidcCallbackSink, but
IdentityModel.OidcClient.LoginAsync still had no IBrowser to drive
the user-agent half of the flow. Without it, the desktop / browser
projects continue to fail at login with 'No browser is available'.

Android now plugs in Chrome Custom Tabs:

  * PostIt.Android/Services/AndroidSystemBrowser.cs implements
    IBrowser.InvokeAsync using CustomTabsIntent.LaunchUrl and waits
    for MainActivity.AndroidOidcCallbackSink to deliver the deep-link
    Intent (android://postit-signin?code=...&state=...).
  * PostIt/Services/Platform.cs is a tiny static indirection the
    shared library uses to ask the running platform for an
    IBrowser and the appropriate default RedirectUri, without
    referencing any UI framework from the shared assembly.
  * LoginPageViewModel reads Platform.DefaultRedirectUri and
    Platform.CreateBrowser().Invoke() before calling LoginAsync.
  * PostIt.Android/PlatformBootstrap.cs wires the Android side at
    startup, and MainActivity.OnCreate calls EnsureInitialized().
  * Xamarin.AndroidX.Browser 1.8.0 added to the central package
    versions so CustomTabsIntent resolves.

src/PostIt/PostIt.Android/MainActivity.cs

@@ -16,6 +16,21 @@ namespace PostIt.Android;
     ConfigurationChanges = ConfigChanges.Orientation | ConfigChanges.ScreenSize | ConfigChanges.UiMode)]
 public class MainActivity : AvaloniaMainActivity
 {
+    /// <summary>
+    /// Strongly-typed handle to the current MainActivity instance, set in
+    /// <see cref="OnCreate"/> and consumed by platform services such as
+    /// <see cref="Services.AndroidSystemBrowser"/> which need to launch
+    /// Chrome Custom Tabs.
+    /// </summary>
+    public static MainActivity? Current { get; private set; }
+
+    protected override void OnCreate(global::Android.OS.Bundle? savedInstanceState)
+    {
+        base.OnCreate(savedInstanceState);
+        PlatformBootstrap.EnsureInitialized();
+        Current = this;
+    }
+
     /// <summary>
     /// Receives the deep-link Intent fired by the system browser after the
     /// user completes the OIDC login on https://yavsc.pschneider.fr. The