feat(konto): settings page for security and privacy

Adds /konto/einstellungen, which gathers two-factor authentication,
"sign out on all devices", the Erntepost consent, the data export
(Art. 15 GDPR) and account deletion (Art. 17 GDPR) in one place.
The account overview now links there through a "Settings" card
instead of listing these blocks inline. Server actions are unchanged
apart from redirecting failures and revalidating the new page.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

src/app/konto/zwei-faktor/page.tsx

@@ -27,7 +27,7 @@ export default async function ZweiFaktorPage() {
     <div className="py-20">
       <Container className="max-w-lg">
         <FormPanel>
-        <BackLink href="/konto" label={t.twoFactorPage.backLabel} />
+        <BackLink href="/konto/einstellungen" label={t.twoFactorPage.backLabel} />
         <SectionHeading
           title={t.twoFactorPage.title}
           subtitle={t.twoFactorPage.subtitle}