feat(design-system): mobile bottom nav + hero copy vertical center

DesignLayout now hides its 260px sidebar below lg and shows a fixed
bottom bar surfacing the current page. Tapping it opens a 50svh bottom
sheet with the full nav list (grouped, scrolls internally) plus the
back-to-site link. Route changes auto-close the sheet and the backdrop
is tap-to-dismiss.

Hero split variant: drop the grid's items-center and add lg:justify-center
on the copy column so the headline/CTA block now lines up with the
image's vertical midpoint instead of sitting at the top.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/i18n/messages.js

@@ -19,6 +19,8 @@ const de = {
   'ds.sidebar.tokens': 'Tokens',
   'ds.sidebar.components': 'Komponenten',
   'ds.sidebar.back': '← Zurück zur Website',
+  'ds.mobile.currentPage': 'Aktuelle Seite',
+  'ds.mobile.selectPage': 'Seite auswählen',
   'ds.nav.logo': 'Logo',
   'ds.nav.colors': 'Farben',
   'ds.nav.typography': 'Typografie',
@@ -246,6 +248,8 @@ const en = {
   'ds.sidebar.tokens': 'Tokens',
   'ds.sidebar.components': 'Components',
   'ds.sidebar.back': '← Back to site',
+  'ds.mobile.currentPage': 'Current page',
+  'ds.mobile.selectPage': 'Select a page',
   'ds.nav.logo': 'Logo',
   'ds.nav.colors': 'Colors',
   'ds.nav.typography': 'Typography',