Add home hero with intro and buttons from the body, next-event card and page tiles

Elternbeirat.Web/Shared/BerlinClock.cs

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+namespace Elternbeirat.Web.Shared;
+
+/// <summary>
+///     Tells the current calendar day as people at the school see it.
+/// </summary>
+/// <remarks>
+///     The container's clock runs in UTC, so <see cref="DateTime.Today"/> would flip to
+///     the next day at 01:00 or 02:00 Berlin time, and an event shortly after midnight
+///     would count as past for an hour or two. Event times are Berlin wall-clock
+///     values (see <c>LocalDateTimeConverter</c>), so "today" has to be a Berlin day
+///     too.
+///     <para>
+///         The time comes from an injected <see cref="TimeProvider"/>, not the system
+///         clock, so tests can pin "today" and check what a page shows before and after
+///         an event.
+///     </para>
+/// </remarks>
+public static class BerlinClock
+{
+    /// <summary>
+    ///     IANA id; resolves on every platform .NET supports, Windows included.
+    /// </summary>
+    private static readonly TimeZoneInfo Berlin =
+        TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");
+
+    /// <summary>
+    ///     Returns today's date in Berlin.
+    /// </summary>
+    /// <param name="time">The clock to read, usually <see cref="TimeProvider.System"/>.</param>
+    /// <returns>The current Berlin calendar day.</returns>
+    public static DateOnly Today(TimeProvider time) =>
+        DateOnly.FromDateTime(TimeZoneInfo.ConvertTime(time.GetUtcNow(), Berlin).DateTime);
+}