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/Directory.Packages.props

@@ -13,6 +13,7 @@
     <PackageVersion Include="Avalonia.Themes.Fluent" Version="12.0.4" />
     <PackageVersion Include="AvaloniaUI.DiagnosticsSupport" Version="2.2.2" />
     <PackageVersion Include="CommunityToolkit.Mvvm" Version="8.4.2" />
+    <PackageVersion Include="Xamarin.AndroidX.Browser" Version="1.8.0" />
     <PackageVersion Include="Xamarin.AndroidX.Core.SplashScreen" Version="1.2.0" />
   </ItemGroup>
 </Project>

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

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);
+        };
+    }
+}

src/PostIt/PostIt.Android/PostIt.Android.csproj

@@ -21,6 +21,7 @@
   <ItemGroup>
     <PackageReference Include="Avalonia.Android" />
     <PackageReference Include="Xamarin.AndroidX.Core.SplashScreen" />
+    <PackageReference Include="Xamarin.AndroidX.Browser" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\PostIt\PostIt.csproj" />

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
+        };
+    }
+}

src/PostIt/PostIt/Services/Platform.cs

@@ -0,0 +1,28 @@
+using IdentityModel.OidcClient.Browser;
+
+namespace PostIt.Services;
+
+/// <summary>
+/// Per-platform access to native integration points used by the OIDC
+/// Authorization Code + PKCE flow. The shared <c>PostIt</c> library does
+/// not reference any UI framework; platform projects (PostIt.Android,
+/// PostIt.Desktop, PostIt.Browser) populate this class once at startup so
+/// the shared <c>LoginPageViewModel</c> can drive a native browser without
+/// taking a hard dependency on any specific UI toolkit.
+/// </summary>
+public static class Platform
+{
+    /// <summary>
+    /// Default redirect URI for the running platform. The desktop loopback
+    /// default is set here; platform projects override this property at
+    /// startup (e.g. PostIt.Android sets it to <c>android://postit-signin</c>).
+    /// </summary>
+    public static string DefaultRedirectUri { get; set; } = "http://127.0.0.1:7890/";
+
+    /// <summary>
+    /// Constructs a fresh <see cref="IBrowser"/> for the running platform.
+    /// May return <c>null</c> if no browser is wired up; in that case
+    /// <c>LoginAsync</c> will surface a clear error.
+    /// </summary>
+    public static System.Func<IBrowser?>? CreateBrowser { get; set; }
+}

src/PostIt/PostIt/ViewModels/LoginPageViewModel.cs

@@ -1,5 +1,6 @@
 using CommunityToolkit.Mvvm.Input;
 using IdentityModel.OidcClient;
+using PostIt.Services;
 using System;
 using System.Threading.Tasks;
 
@@ -38,7 +39,20 @@ public partial class LoginPageViewModel : ViewModelBase
         {
             Settings.Load().Wait();
 
-            var client = new OidcClient(Settings.GetOidcClientOptions());
+            // The platform project picks the right redirect URI and browser
+            // implementation; we don't reference any UI toolkit from here.
+            Settings.RedirectUri = string.IsNullOrWhiteSpace(Settings.RedirectUri)
+                ? Platform.DefaultRedirectUri
+                : Settings.RedirectUri;
+
+            var browser = Platform.CreateBrowser?.Invoke();
+            if (browser is null)
+            {
+                StatusMessage = "No browser is available on this platform.";
+                return;
+            }
+
+            var client = new OidcClient(Settings.GetOidcClientOptions(browser));
             var loginResult = await client.LoginAsync(new LoginRequest());
 
             if (loginResult.IsError)