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/NewsletterModal.tsx

@@ -34,13 +34,40 @@ export function NewsletterModal({ open, onClose }: { open: boolean; onClose: ()
   const dialogRef = useRef<HTMLDivElement>(null);
   const closeButtonRef = useRef<HTMLButtonElement>(null);
 
-  // Body scroll lock while open.
+  // Background scroll lock while open — intercepts and cancels the wheel/
+  // touch input that would cause scrolling, instead of toggling
+  // overflow/position on body or html. Two earlier approaches (plain
+  // overflow:hidden on body; position:fixed with a negative top offset to
+  // compensate) each fixed one symptom while causing another — overflow
+  // alone reset scrollY to 0 for a frame when opened away from the top of
+  // the page (e.g. parked at #ueber-bjoern), and position:fixed took body
+  // out of normal flow, which detached the Navbar's `position: sticky`
+  // from its scrolling container and made it visibly snap. Neither is a
+  // risk here: scrollY, overflow and layout are never touched at all, so
+  // there's nothing that can jump, reflow, or need restoring on close —
+  // the events that would move the page just never get to.
+  // `{ passive: false }` is required for preventDefault() to have any
+  // effect on wheel/touchmove. Events that originate inside the dialog
+  // (which has its own overflow-y-auto) are let through untouched, so the
+  // modal's own content still scrolls normally.
   useEffect(() => {
     if (!open) return;
-    const prevOverflow = document.body.style.overflow;
-    document.body.style.overflow = "hidden";
+
+    const isInsideDialog = (target: EventTarget | null) =>
+      target instanceof Node && !!dialogRef.current?.contains(target);
+
+    const onWheel = (e: WheelEvent) => {
+      if (!isInsideDialog(e.target)) e.preventDefault();
+    };
+    const onTouchMove = (e: TouchEvent) => {
+      if (!isInsideDialog(e.target)) e.preventDefault();
+    };
+
+    document.addEventListener("wheel", onWheel, { passive: false });
+    document.addEventListener("touchmove", onTouchMove, { passive: false });
     return () => {
-      document.body.style.overflow = prevOverflow;
+      document.removeEventListener("wheel", onWheel);
+      document.removeEventListener("touchmove", onTouchMove);
     };
   }, [open]);
 
@@ -50,7 +77,16 @@ export function NewsletterModal({ open, onClose }: { open: boolean; onClose: ()
   useEffect(() => {
     if (!open) return;
 
-    closeButtonRef.current?.focus();
+    // preventScroll: true — otherwise the browser's implicit
+    // scrollIntoView on focus() sees this button sitting close to the
+    // viewport's top edge and "corrects" for the global
+    // scroll-padding-top (reserved for the sticky Navbar, see
+    // globals.css) by nudging the whole page upward — pointless here
+    // since the modal is position:fixed and always fully in view
+    // regardless of document scroll, but that nudge is exactly the
+    // "page scrolls up a bit and the modal lands in a weird position"
+    // jank on open.
+    closeButtonRef.current?.focus({ preventScroll: true });
 
     const onKeyDown = (e: KeyboardEvent) => {
       if (e.key === "Escape") {
@@ -67,10 +103,10 @@ export function NewsletterModal({ 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 });
       }
     };