postit: point Register button at /Account/Register

/signin only renders the local-account sign-in form on Yavsc.Org;
new accounts live at /Account/Register. Update RegisterUrl in
LoginPageViewModel and the matching test accordingly.

src/PostIt.Tests/SettingsLoadTests.cs

@@ -0,0 +1,36 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace PostIt.Tests;
+
+public class SettingsLoadTests
+{
+    /// <summary>
+    /// On the dev machine, the user-level settings file
+    /// (~/.config/PostIt/postit-settings.json) does not exist, so Load()
+    /// must fall back to the embedded default resource shipped inside
+    /// PostIt.dll.
+    /// </summary>
+    [Fact]
+    public async Task Load_falls_back_to_embedded_resource_when_user_file_missing()
+    {
+        // Skip if a user-level file exists (CI / different dev machines).
+        var userConfigPath = Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "PostIt",
+            "postit-settings.json");
+        if (File.Exists(userConfigPath))
+        {
+            return; // nothing to assert: user file wins.
+        }
+
+        var settings = new PostIt.Settings();
+        await settings.Load();
+
+        // The bundled postit-settings.json points at yavsc.pschneider.fr.
+        Assert.False(string.IsNullOrWhiteSpace(settings.Authentication?.Authority));
+        Assert.Equal("postit", settings.Authentication.ClientId);
+    }
+}