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/PlatformBootstrap.cs

@@ -0,0 +1,29 @@
+using PostIt.Services;
+using PostIt.Android.Services;
+
+namespace PostIt.Android;
+
+/// <summary>
+/// One-shot platform bootstrap. Called from
+/// <see cref="MainActivity.OnCreate"/> so that the shared
+/// <c>LoginPageViewModel</c> sees the Android-specific redirect URI and a
+/// working <c>IBrowser</c> (Chrome Custom Tabs) without referencing
+/// Android APIs from the shared library.
+/// </summary>
+internal static class PlatformBootstrap
+{
+    private static int _initialized;
+
+    internal static void EnsureInitialized()
+    {
+        if (System.Threading.Interlocked.Exchange(ref _initialized, 1) != 0)
+            return;
+
+        Platform.DefaultRedirectUri = Settings.AndroidRedirectUri;
+        Platform.CreateBrowser = () =>
+        {
+            var activity = MainActivity.Current;
+            return activity is null ? null : new AndroidSystemBrowser(activity);
+        };
+    }
+}