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/IframeBlock.stories.ts

@@ -0,0 +1,22 @@
+import type { Meta, StoryObj } from '@storybook/sveltekit';
+import IframeBlock from './IframeBlock.svelte';
+
+const meta = {
+  title: 'CMS-Blocks/Iframe',
+  component: IframeBlock,
+  tags: ['autodocs'],
+  argTypes: { block: { control: 'object' } },
+} satisfies Meta<typeof IframeBlock>;
+
+export default meta;
+type Story = StoryObj<typeof meta>;
+
+export const OpenStreetMap: Story = {
+  args: {
+    block: {
+      iframe:
+        '<iframe width="100%" height="400" frameborder="0" src="https://www.openstreetmap.org/export/embed.html?bbox=10.4,49.9,10.7,50.1&layer=mapnik"></iframe>',
+      content: 'Kartenausschnitt Steigerwald.',
+    },
+  },
+};