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/Card.stories.svelte

@@ -0,0 +1,24 @@
+<script module lang="ts">
+  import { defineMeta } from '@storybook/addon-svelte-csf';
+  import Card from './Card.svelte';
+
+  const { Story } = defineMeta({
+    title: 'Molecules/Card',
+    component: Card,
+    tags: ['autodocs'],
+  });
+</script>
+
+<Story name="Tile">
+  <div class="p-4">
+    <h3 class="font-semibold">Karten-Titel</h3>
+    <p class="text-sm text-stein-600">Inhalt der Kachel.</p>
+  </div>
+</Story>
+
+<Story name="AlsLink" args={{ href: '#', variant: 'tile' }}>
+  <div class="p-4">
+    <h3 class="font-semibold">Klickbare Karte</h3>
+    <p class="text-sm text-stein-600">Ganze Karte ist ein Link.</p>
+  </div>
+</Story>