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

Elternbeirat.Web/Features/Embeds/PostsEmbed.razor.css

@@ -1,5 +1,5 @@
-/* Card styles for .post-list are shared with the full posts page and live in the
-   global app.css. Only the embed's spacing on the home page is specific here. */
+/* The card is PostCard and the list grid lives in the global app.css, shared with
+   the full posts page. Only the embed's spacing on the home page is specific here. */
 .embed-posts {
     margin-top: var(--space-5);
 }