feat(design-system): Storybook + Button-Atom-Vereinheitlichung + SectionHeader

Storybook (SvelteKit-Framework, Tailwind v4, MSW, Translation-Decorator):
Stories für Atoms/Molecules/Organisms + alle CMS-Blocks.

Button-Atom: 5 .btn-*-Varianten (primary/secondary/tertiary/outline/warning),
class-Merge-Prop, href→<a>, Loading-Spinner neben Label, rounded-md/font-medium.
Alle rohen .btn-*- und Hand-Style-Buttons app-weit aufs Atom migriert.

SectionHeader-Molecule (kicker/number/title/subtitle/action) + Integration in
7 CMS-Blocks (post_overview, organisations, organisations_map, searchable_text,
image_gallery, youtube_video_gallery, files) via neuem section_header-Schema.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/blocks/YoutubeVideoGalleryBlock.svelte

@@ -2,6 +2,7 @@
   import { marked } from "$lib/markdown-safe";
   import { fade } from "svelte/transition";
   import { getBlockLayoutClasses } from "$lib/block-layout";
+  import SectionHeader from "$lib/components/SectionHeader.svelte";
   import type {
     YoutubeVideoGalleryBlockData,
     YoutubeVideoBlockData,
@@ -72,6 +73,9 @@
   }
 
   const layoutClasses = $derived(getBlockLayoutClasses(block.layout));
+  const actionRef = $derived(
+    block.action && typeof block.action === "object" ? block.action : null,
+  );
   const descriptionHtml = $derived(
     block.description && typeof block.description === "string"
       ? (marked.parse(block.description) as string)
@@ -149,10 +153,17 @@
   data-block-slug={block._slug}
 >
   {#if block.title}
-    <h3 class="text-lg font-semibold text-stein-900 mb-1">{block.title}</h3>
-  {/if}
-  {#if block.subtitle}
-    <p class="text-sm text-stein-600 mb-3">{block.subtitle}</p>
+    <div class="mb-3">
+      <SectionHeader
+        number={block.number}
+        kicker={block.kicker}
+        title={block.title}
+        subtitle={block.subtitle}
+        tag="h3"
+        actionLabel={actionRef?.linkName}
+        actionHref={actionRef?.url}
+      />
+    </div>
   {/if}
   {#if descriptionHtml}
     <div class="markdown max-w-none prose prose-zinc mb-4">