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/Footer.tsx

@@ -33,12 +33,15 @@ export function Footer() {
           </div>
 
           {/* Social handle — Lora Regular (weight 400) */}
-          <p
-            className="font-normal text-bg-white text-h-small text-center leading-[1.2] whitespace-nowrap shrink-0"
+          <a
+            href="https://www.instagram.com/einfach.produktiv/"
+            target="_blank"
+            rel="noopener noreferrer"
+            className="font-normal text-bg-white text-h-small text-center leading-[1.2] whitespace-nowrap shrink-0 hover:text-brand transition-colors"
             style={{ fontFamily: "var(--font-lora)", fontWeight: 400 }}
           >
             @einfach.produktiv
-          </p>
+          </a>
 
           {/* Legal links — wrap + center below md instead of a rigid row */}
           <div className="flex flex-wrap items-center justify-center gap-x-6 gap-y-2 shrink-0">