fix: resolve HTML hierarchy, accessibility and PDF link issues

- Fix heading level skip in ContactContent success state (H3 -> H2)
- Clean up deprecated iframe attributes in ContactContent
- Add aria-label to footer logo link for WCAG compliance
- Rename PDF assets to hyphenated filenames to fix link checks
- Update AGB page with new PDF link format

components/Layout.tsx

@@ -184,7 +184,11 @@ const Layout = ({ children }: { children: React.ReactNode }) => {
           <div className="container-custom relative z-10">
             <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-4 gap-10 md:gap-12 mb-12 md:mb-16">
               <div className="lg:col-span-2">
-                <Link href="/" className="inline-block mb-6 md:mb-8 group">
+                <Link
+                  href="/"
+                  className="inline-block mb-6 md:mb-8 group"
+                  aria-label={t("nav.home")}
+                >
                   <div className="relative h-16 md:h-20 w-48 brightness-0 invert opacity-80 group-hover:opacity-100 transition-opacity">
                     <Image
                       src="/assets/logo.png"