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>

.storybook/decorators/WithTranslations.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+  import type { Snippet } from 'svelte';
+  import TranslationProvider from '$lib/components/TranslationProvider.svelte';
+  import strings from '$lib/components/blocks/_fixtures/translations.json';
+
+  // Storybook reicht die Story als `children`-Snippet rein. Wir setzen den
+  // Translation-Context (echte deutsche Strings aus dem CMS-Bundle), damit
+  // Blocks lesbare Texte statt roher Keys zeigen.
+  let { children }: { children: Snippet } = $props();
+</script>
+
+<TranslationProvider translations={strings}>
+  {@render children()}
+</TranslationProvider>