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

Elternbeirat.Web/Features/Posts/PostList.razor.css

@@ -1,2 +1,12 @@
-/* The .post-list card styles are shared with the home-page embed and live in the
-   global app.css. This page needs no extra styling of its own. */
+/* The card is PostCard and the base list grid lives in the global app.css, shared
+   with the home-page embed. Only the page's two-column layout is here: one column
+   on a phone, two from tablet width, with the newest post across both. */
+@media (min-width: 48rem) {
+    .post-list-grid {
+        grid-template-columns: repeat(2, minmax(0, 1fr));
+    }
+
+    .post-list-featured {
+        grid-column: 1 / -1;
+    }
+}