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/SectionHeader.stories.ts

@@ -0,0 +1,44 @@
+import type { Meta, StoryObj } from '@storybook/sveltekit';
+import SectionHeader from './SectionHeader.svelte';
+
+const meta = {
+  title: 'Molecules/SectionHeader',
+  component: SectionHeader,
+  tags: ['autodocs'],
+  argTypes: {
+    number: { control: 'text' },
+    kicker: { control: 'text' },
+    title: { control: 'text' },
+    subtitle: { control: 'text' },
+    actionLabel: { control: 'text' },
+    actionHref: { control: 'text' },
+    tag: { control: 'select', options: ['h1', 'h2', 'h3'] },
+  },
+} satisfies Meta<typeof SectionHeader>;
+
+export default meta;
+type Story = StoryObj<typeof meta>;
+
+export const MitAktion: Story = {
+  args: {
+    number: '03',
+    kicker: 'Vor Ort',
+    title: 'Beteiligte Initiativen',
+    subtitle: '13 Bürgerinitiativen im Bündnis — direkt vor Ort ansprechbar.',
+    actionLabel: 'Zum Adressbuch',
+    actionHref: '/adressbuch',
+  },
+};
+
+export const OhneAktion: Story = {
+  args: { number: '01', kicker: 'Über uns', title: 'Wer sind wir?' },
+};
+
+export const OhneNummer: Story = {
+  args: {
+    kicker: 'Aktuelles',
+    title: 'Die letzten Beiträge',
+    actionLabel: 'Alle Beiträge',
+    actionHref: '/aktuelles',
+  },
+};