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/OpnFormBlock.stories.ts

@@ -0,0 +1,24 @@
+import type { Meta, StoryObj } from '@storybook/sveltekit';
+import OpnFormBlock from './OpnFormBlock.svelte';
+
+// Bettet ein OpnForm per iframe ein (externer Dienst opnform.pm86.de). Lädt in
+// Storybook nur bei Netzzugriff; die Struktur/Layout ist trotzdem prüfbar.
+const meta = {
+  title: 'CMS-Blocks/OpnForm',
+  component: OpnFormBlock,
+  tags: ['autodocs'],
+  argTypes: { block: { control: 'object' } },
+} satisfies Meta<typeof OpnFormBlock>;
+
+export default meta;
+type Story = StoryObj<typeof meta>;
+
+export const Standard: Story = {
+  args: {
+    block: {
+      formId: 'kontakt',
+      baseUrl: 'https://opnform.pm86.de',
+      iframeTitle: 'Kontaktformular',
+    },
+  },
+};