Add /widerruf and /agb legal pages, fix newsletter modal scroll jank

- New /widerruf page: CMS-driven content via getLegalPage("widerruf"),
  sidebar TOC + "Nachhaltig handeln" callout, TrustRow, and a download
  card for the Muster-Widerrufsformular PDF attachment.
- New /agb page, same legal-page shell pattern as Impressum/Datenschutz.
- lib/payload.ts: getLegalPage now also resolves the optional `attachment`
  media relation (depth=1) for legal pages that have a downloadable file.
- NewsletterModal: replaced the body overflow/position scroll-lock hacks
  (which fought with the sticky Navbar and the global scroll-smooth CSS)
  with a wheel/touch event interceptor that never touches scroll position
  at all, plus `preventScroll: true` on all modal/drawer focus() calls to
  stop the browser's implicit scrollIntoView from nudging the page for
  elements near the sticky navbar's reserved scroll-padding-top zone.
  Same preventScroll fix applied to VersandModal and the Navbar mobile
  drawer for consistency.
- globals.css: scrollbar-gutter: stable, so a modal's overflow:hidden
  lock never causes a scrollbar-width layout shift.
- Divider: stagger its reveal animation slightly (delay 0.3s) behind the
  section above it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/components/VersandModal.tsx

@@ -26,7 +26,7 @@ export function VersandModal({ open, onClose }: { open: boolean; onClose: () =>
   useEffect(() => {
     if (!open) return;
 
-    closeButtonRef.current?.focus();
+    closeButtonRef.current?.focus({ preventScroll: true });
 
     const onKeyDown = (e: KeyboardEvent) => {
       if (e.key === "Escape") {
@@ -43,10 +43,10 @@ export function VersandModal({ open, onClose }: { open: boolean; onClose: () =>
       const last = focusables[focusables.length - 1];
       if (e.shiftKey && document.activeElement === first) {
         e.preventDefault();
-        last.focus();
+        last.focus({ preventScroll: true });
       } else if (!e.shiftKey && document.activeElement === last) {
         e.preventDefault();
-        first.focus();
+        first.focus({ preventScroll: true });
       }
     };