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/globals.css

@@ -141,6 +141,12 @@
 
 html {
   scroll-padding-top: 6.25rem; /* navbar height */
+  /* Reserves the scrollbar's width permanently, so any modal's
+     overflow:hidden scroll lock (NewsletterModal, VersandModal) never
+     removes/re-adds the scrollbar itself — without this, losing the
+     scrollbar on open widens the viewport by its width and every fixed/sticky
+     element (Navbar included) visibly snaps sideways for one frame. */
+  scrollbar-gutter: stable;
 }
 
 body {