Add App__Domain config to derive connect redirect URI

Let deployments set a single App__Domain (e.g. sptb.example.com) instead of
spelling out the full ClientConnect__RedirectUri. The SharePoint-connect
callback is derived as <domain>/connect/callback; an explicit RedirectUri
still wins for back-compat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Core/Config/AppDomainOptions.cs

@@ -0,0 +1,29 @@
+namespace SharepointToolbox.Web.Core.Config;
+
+/// <summary>
+/// The app's public domain (e.g. <c>sptb.example.com</c> or <c>https://sptb.example.com</c>),
+/// configured via <c>App__Domain</c>. Used to derive the SharePoint-connect redirect URI when
+/// <see cref="ClientConnectOptions.RedirectUri"/> isn't set explicitly.
+/// </summary>
+public class AppDomainOptions
+{
+    public string Domain { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Builds an absolute URL for <paramref name="path"/> rooted at the configured domain, or
+    /// <c>null</c> when no domain is set. Defaults to <c>https</c> when the domain has no scheme,
+    /// and tolerates accidental surrounding quotes / trailing slashes (docker-compose's list-form
+    /// env values can embed literal quotes).
+    /// </summary>
+    public string? BuildUrl(string path)
+    {
+        var domain = Domain?.Trim().Trim('"', '\'').TrimEnd('/');
+        if (string.IsNullOrEmpty(domain))
+            return null;
+
+        if (!domain.Contains("://", StringComparison.Ordinal))
+            domain = "https://" + domain;
+
+        return domain + "/" + path.TrimStart('/');
+    }
+}