refactor(consolidate): ArrowLink tone + FormSuccess-Komponente

A: ArrowLink um tone-Prop (himmel/wald/stein/white) — PostOverview + CalendarCompact
inline Arrow-Links → ArrowLink.
B: 3× identische Formular-Success-Boxen → internal/FormSuccess.svelte (rounded-lg).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/ArrowLink.stories.ts

@@ -9,6 +9,7 @@ const meta = {
   args: { href: '#', label: 'Mehr erfahren' },
   argTypes: {
     size: { control: 'select', options: ['sm', 'md'] },
+    tone: { control: 'select', options: ['himmel', 'wald', 'stein', 'white'] },
     external: { control: 'boolean' },
   },
 } satisfies Meta<typeof ArrowLink>;
@@ -19,3 +20,5 @@ type Story = StoryObj<typeof meta>;
 export const Medium: Story = { args: { size: 'md', label: 'Zum Adressbuch' } };
 export const Klein: Story = { args: { size: 'sm', label: 'cloud.windwiderstand.de' } };
 export const Extern: Story = { args: { size: 'sm', label: 'Beitreten', external: true } };
+export const Wald: Story = { args: { tone: 'wald', label: 'Alle Termine' } };
+export const Stein: Story = { args: { tone: 'stein', size: 'sm', label: 'Alle Beiträge' } };