Rework post page (lead, pager), add skip link, unified focus ring, reduced motion and keyboard-accessible menu

Elternbeirat.Web/Features/Posts/PostDetail.razor.cs

@@ -32,6 +32,10 @@ public partial class PostDetail
 
     private Post? _post;
 
+    // The older and newer post for the "Vorheriger / Nächster Beitrag" links at the
+    // end; both empty until loaded or when the post stands alone.
+    private PostNeighbors _neighbors = new(null, null);
+
     // Set when the content source could not be reached or read. The markup then
     // shows a "temporarily unavailable" note instead of a not-found page, matching
     // the 503 this sets on the response.
@@ -48,6 +52,7 @@ public partial class PostDetail
             var posts = await PocketBase.GetPostsAsync(HttpContext?.RequestAborted ?? CancellationToken.None);
             _post = posts.FirstOrDefault(
                 post => string.Equals(post.Slug, Slug, StringComparison.OrdinalIgnoreCase));
+            _neighbors = PostNeighbors.Of(posts, Slug);
         }
         catch (PocketBaseUnavailableException exception)
         {