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/Services/AndroidSystemBrowser.cs

@@ -0,0 +1,83 @@
+using System;
+using System.Threading.Tasks;
+using Android.App;
+using Android.Content;
+using AndroidX.Browser.CustomTabs;
+using IdentityModel.OidcClient.Browser;
+
+namespace PostIt.Android.Services;
+
+/// <summary>
+/// <see cref="IBrowser"/> implementation that drives Chrome Custom Tabs for
+/// the OIDC Authorization Code + PKCE flow. The identity provider redirects
+/// to <c>android://postit-signin?code=...&amp;state=...</c>, which Android
+/// routes back to the running PostIt instance via the activity-alias
+/// declared in <c>AndroidManifest.xml</c>; the resulting Intent URI is
+/// handed back through <see cref="MainActivity.AndroidOidcCallbackSink"/>.
+/// </summary>
+public sealed class AndroidSystemBrowser : IBrowser
+{
+    private readonly Activity _activity;
+
+    public AndroidSystemBrowser(Activity activity)
+    {
+        _activity = activity ?? throw new ArgumentNullException(nameof(activity));
+    }
+
+    public async Task<BrowserResult> InvokeAsync(BrowserOptions options, System.Threading.CancellationToken cancellationToken = default)
+    {
+        if (options is null) throw new ArgumentNullException(nameof(options));
+        if (string.IsNullOrWhiteSpace(options.StartUrl))
+        {
+            return new BrowserResult
+            {
+                ResultType = BrowserResultType.UnknownError,
+                Error = "BrowserOptions.StartUrl is empty."
+            };
+        }
+
+        var uri = global::Android.Net.Uri.Parse(options.StartUrl)!;
+
+        var callbackTask = MainActivity.AndroidOidcCallbackSink.AwaitNextCallbackAsync();
+
+        var tabsIntent = new CustomTabsIntent.Builder()
+            .SetShowTitle(true)
+            .Build();
+        tabsIntent.LaunchUrl(_activity, uri);
+
+        string responseUri;
+        try
+        {
+            responseUri = await callbackTask.WaitAsync(cancellationToken).ConfigureAwait(true);
+        }
+        catch (OperationCanceledException)
+        {
+            return new BrowserResult
+            {
+                ResultType = BrowserResultType.UserCancel
+            };
+        }
+        catch (Exception ex)
+        {
+            return new BrowserResult
+            {
+                ResultType = BrowserResultType.UnknownError,
+                Error = $"Failed to await OIDC callback: {ex.Message}"
+            };
+        }
+
+        if (string.IsNullOrEmpty(responseUri))
+        {
+            return new BrowserResult
+            {
+                ResultType = BrowserResultType.UserCancel
+            };
+        }
+
+        return new BrowserResult
+        {
+            ResultType = BrowserResultType.Success,
+            Response = responseUri
+        };
+    }
+}