postit: test LoginPageViewModel against an in-memory OIDC authority

Add a stubbed OIDC authority (discovery, jwks, /connect/token,
/connect/userinfo) and a fake IBrowser, then cover the full
authorization-code + PKCE flow in LoginPageViewModelTests.

The new LoginPageViewModel(Settings, Func<IBrowser?>) constructor
plus the BrowserFactoryOverride property keep production wiring
unchanged: the existing parameterless ctor and the platform
projects' Platform.CreateBrowser still drive runtime.

src/PostIt.Tests/LoginPageViewModelTests.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Threading.Tasks;
+using PostIt.ViewModels;
+using Xunit;
+
+namespace PostIt.Tests;
+
+public class LoginPageViewModelTests
+{
+    [Fact]
+    public async Task LoginAsync_acquires_access_token_from_stubbed_yavsc_authority()
+    {
+        // Arrange: spin up a stub OIDC authority and a fake browser that
+        // short-circuits the system browser. The authority signs its
+        // access_token with RS256; the fake browser captures the redirect
+        // URI so the authority can complete the token exchange.
+        using var authority = await OidcStubAuthority.StartAsync();
+        var browser = new FakeAuthorizingBrowser(authority.LoopbackRedirectUri);
+
+        var settings = new PostIt.Settings
+        {
+            Authentication = new AuthenticationSettings
+            {
+                Authority = authority.Issuer,
+                ClientId = "postit-tests"
+            },
+            RedirectUri = authority.LoopbackRedirectUri,
+            Scopes = new[] { "openid", "profile", "blog" }
+        };
+
+        var vm = new LoginPageViewModel(settings, browser.CreateBrowser);
+
+        // Act
+        await vm.LoginAsync();
+
+        // Assert: the ViewModel surfaced a token, not an error.
+        Assert.True(
+            !string.IsNullOrEmpty(vm.AccessToken),
+            $"Login did not produce a token. StatusMessage={vm.StatusMessage ?? "<null>"}");
+        Assert.False(
+            vm.StatusMessage?.StartsWith("Error") == true,
+            $"Login reported error: {vm.StatusMessage}");
+    }
+}