feat: Dark Mode + Zertifikat-Verwaltung im Superadmin

- Dark Mode: ThemeProvider (next-themes), ThemeToggle in Navbar (Hell/Dunkel/System)
- Zertifikat-Tab (superadmin only): aktuelles Zertifikat anzeigen, Upload (cert+key),
  Self-Signed ausstellen, Let's Encrypt/ACME
- Backend: /api/admin/cert/* Endpunkte (info, upload, self-signed, acme), nginx reload
- HTTPS bereits live auf Server (self-signed RSA-4096, Port 443)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/theme-provider.tsx

@@ -0,0 +1,16 @@
+"use client"
+
+import { ThemeProvider as NextThemesProvider } from "next-themes"
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  return (
+    <NextThemesProvider
+      attribute="class"
+      defaultTheme="system"
+      enableSystem
+      disableTransitionOnChange
+    >
+      {children}
+    </NextThemesProvider>
+  )
+}