feat(calendar): markdown descriptions, grid & header polish

- description field rendered as Markdown (marked.parse + prose)
- month label left/larger, nav arrows grouped right
- weekday headers colored (himmel/stein)
- day cells weekend/weekday coloring
- remove pl-[4.75rem] indent from accordion body

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/components/blocks/CalendarBlock.svelte

@@ -12,6 +12,7 @@
         type CalendarItemICS,
     } from "$lib/calendar-ics";
     import { getCmsImageUrl, extractCmsImageField } from "$lib/rusty-image";
+    import { marked } from "$lib/markdown-safe";
     import "$lib/iconify-offline";
     import Icon from "@iconify/svelte";
     import ImageModal from "$lib/components/ImageModal.svelte";
@@ -686,7 +687,7 @@
                     />
                 </summary>
                 <div
-                    class="px-3 pb-3 pt-0 pl-[4.75rem] text-sm text-stein-700 space-y-2"
+                    class="px-3 pb-3 pt-0 text-sm text-stein-700 space-y-2"
                 >
                     <div
                         class="text-[11px] uppercase tracking-wide text-stein-500 -mt-1"
@@ -694,11 +695,9 @@
                         {relativeDays(item.start)}
                     </div>
                     {#if item.description}
-                        <p
-                            class="font-light text-stein-700 m-0! leading-relaxed"
-                        >
-                            {item.description}
-                        </p>
+                        <div class="prose prose-sm prose-stein max-w-none font-light leading-relaxed mt-0 mb-2">
+                            {@html marked.parse(item.description)}
+                        </div>
                     {/if}
                     {#if item.tags && item.tags.length > 2}
                         <div class="flex flex-wrap gap-1">