feat(layout): migrate to full-width topbar with mobile drawer navigation

Move topbar outside the grid container as a standalone sticky header
(Shopify/Stripe pattern). Sidebar and icon bar use position:sticky
relative to topbar height. Mobile uses drawer pattern with backdrop,
focus trap, ESC close, and scroll lock. Tenant branding moved from
sidebar header into the topbar.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

i18n/default_de.json

@@ -1435,5 +1435,7 @@
   "Remove selected file": "Ausgewaehlte Datei entfernen",
   "Replace": "Ersetzen",
   "Delete this image?": "Dieses Bild löschen?",
-  "Current image": "Aktuelles Bild"
+  "Current image": "Aktuelles Bild",
+  "Open navigation": "Navigation öffnen",
+  "Close navigation": "Navigation schließen"
 }