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

Elternbeirat.Web/Features/Posts/PostCard.razor.cs

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+using Elternbeirat.Contracts;
+using Elternbeirat.Web.Shared;
+using Microsoft.AspNetCore.Components;
+
+namespace Elternbeirat.Web.Features.Posts;
+
+/// <summary>
+///     Renders one post as a card linking to the post, with date, title and teaser.
+/// </summary>
+/// <remarks>
+///     Shared by the posts page and the home-page posts embed, so both show posts the
+///     same way. The teaser comes from the body (<see cref="Markdown.Teaser"/>): posts
+///     have no summary field of their own.
+/// </remarks>
+/// <seealso cref="PostDate"/>
+public partial class PostCard
+{
+    /// <summary>
+    ///     The post to show.
+    /// </summary>
+    [Parameter]
+    [EditorRequired]
+    public Post Value { get; set; } = null!;
+
+    /// <summary>
+    ///     <see langword="true"/> for the large card that opens the posts page (the
+    ///     newest post); <see langword="false"/> for a regular card.
+    /// </summary>
+    [Parameter]
+    public bool Featured { get; set; }
+
+    /// <summary>
+    ///     The card's classes; the featured card adds its modifier.
+    /// </summary>
+    private string CssClass => Featured ? "post-card post-card-featured" : "post-card";
+
+    /// <summary>
+    ///     The address of the post.
+    /// </summary>
+    private string Href => $"/posts/{Value.Slug}";
+
+    /// <summary>
+    ///     The teaser text; empty if the body has no paragraph text.
+    /// </summary>
+    private string Teaser => Markdown.Teaser(Value.Body);
+}