feat(15-06): phase-wide i18n keys + sixth admin nav entry for groups

- de.json/en.json: admin.groups.* (incl. ldapBind, members, deleteConfirm,
  grants sub-namespaces), admin.users.grants.*, adminModules.grantsLink +
  grants.* + activationDialog.*, modules.accessDenied.*, marketplace
  statusNoAccess/toastNoAccess, header.admin.groups -- covers this plan's
  /admin/groups surface plus the Wave 4 surfaces (permission matrix,
  user-detail grants, activation dialog, 403 page, marketplace badge) so
  15-07/15-08 can run in parallel without touching the translation files
- admin-sidebar.tsx: sixth nav entry "Gruppen" -> /admin/groups with a
  roster/list icon (Lucide list glyph, distinct from the users icon)

apps/web/src/components/admin/admin-sidebar.tsx

@@ -71,6 +71,21 @@ export function AdminSidebar() {
         </svg>
       ),
     },
+    {
+      label: t('admin.groups'),
+      href: '/admin/groups',
+      show: true,
+      icon: (
+        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <line x1="8" x2="21" y1="6" y2="6" />
+          <line x1="8" x2="21" y1="12" y2="12" />
+          <line x1="8" x2="21" y1="18" y2="18" />
+          <line x1="3" x2="3.01" y1="6" y2="6" />
+          <line x1="3" x2="3.01" y1="12" y2="12" />
+          <line x1="3" x2="3.01" y1="18" y2="18" />
+        </svg>
+      ),
+    },
   ];
 
   return (