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

Elternbeirat.Web/Features/Events/CalendarSheet.razor

@@ -0,0 +1,10 @@
+@*
+    A tear-off calendar sheet for an event's start: short month on a green band,
+    the day large, the weekday small. Pure decoration, hidden from screen readers:
+    every caller puts the full date (<EventDate>) next to it.
+*@
+<span class="calendar-sheet" aria-hidden="true">
+    <span class="calendar-month">@EventDateFormat.SheetMonth(Value)</span>
+    <span class="calendar-day">@EventDateFormat.SheetDay(Value)</span>
+    <span class="calendar-weekday">@EventDateFormat.SheetWeekday(Value)</span>
+</span>