Initial Blazor Web App template

Rider-generated Blazor Web App (net10.0, interactive render mode None,
no auth scaffold, no sample pages). Includes Rider auto-formatting.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Elternbeirat.Web/Components/Pages/NotFound.razor

@@ -0,0 +1,5 @@
+@page "/not-found"
+@layout MainLayout
+
+<h3>Not Found</h3>
+<p>Sorry, the content you are looking for does not exist.</p>