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

@@ -6,12 +6,7 @@
         <ul class="post-list">
             @foreach (var post in _latest)
             {
-                <li>
-                    <a class="card-link" href="@($"/posts/{post.Slug}")">
-                        <span class="post-title">@post.Title</span>
-                        <PostDate Value="post.Date" />
-                    </a>
-                </li>
+                <li><PostCard Value="post" /></li>
             }
         </ul>
     </section>