Add post and event cards, shared calendar sheet and per-event ICS; rework past events and next-event card

Elternbeirat.Web/Features/Events/EventList.razor.cs

@@ -39,6 +39,10 @@ public partial class EventList
     // 503 this sets on the response.
     private bool _unavailable;
 
+    // Past events grouped by year for the small year headings. _pastEvents is most
+    // recent first, and GroupBy keeps that order, so the newest year comes first.
+    private IEnumerable<IGrouping<int, Event>> PastYears => _pastEvents.GroupBy(@event => @event.Start.Year);
+
     // The page body rendered from Markdown to HTML, ready to emit as a MarkupString.
     private MarkupString Body => (MarkupString)Markdown.ToHtml(_page?.Body);