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/ImageGalleryBlock.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 "$lib/components/SectionHeader.svelte";
   import type { ImageGalleryBlockData, ImageGalleryImage } from "$lib/block-types";
   import "$lib/iconify-offline";
   import Icon from "@iconify/svelte";
@@ -12,6 +13,9 @@
 
   const t = useTranslate();
   let { block }: { block: ImageGalleryBlockData } = $props();
+  const actionRef = $derived(
+    block.action && typeof block.action === "object" ? block.action : null,
+  );
 
   /** Hover-Label aus image-Field — title|alt|caption|description, in dieser Reihenfolge. */
   function imageLabel(img: ImageGalleryImage): string {
@@ -166,7 +170,16 @@
     <p class="text-sm text-stein-500">{t(T.image_gallery_empty)}</p>
   {:else if block.variant === "grid"}
     {#if block.title}
-      <h3 class="text-lg font-semibold text-stein-900 mb-3">{block.title}</h3>
+      <div class="mb-3">
+        <SectionHeader
+          number={block.number}
+          kicker={block.kicker}
+          title={block.title}
+          tag="h3"
+          actionLabel={actionRef?.linkName}
+          actionHref={actionRef?.url}
+        />
+      </div>
     {/if}
     <ul class="not-prose grid grid-cols-2 gap-2 sm:grid-cols-3 md:grid-cols-4">
       {#each withUrl as item, i}