i18n(frontend): translate all German UI strings to English

Replace German labels, button text, toast messages, table headers,
tooltips, and placeholder strings across 7 files:
- WorkflowEditor: buttons, toasts, node labels
- Tenants: buttons, toasts, dialog text, table headers
- Admin: widget layout description
- OrderDetail: column headers (Baureihe→Series, Ebene→Level, Lagertyp→Bearing Type)
- ExcelSpreadsheet: column label definitions
- Upload: series/duplicate warning strings
- TemplateEditor: ALL_FIELD_DEFS default labels

API field names (baureihe, ebene1, produkt_baureihe etc.) unchanged.

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

frontend/src/pages/Admin.tsx

@@ -950,18 +950,18 @@ export default function AdminPage() {
             <div>
               <h2 className="font-semibold text-content">Dashboard Widget-Konfiguration</h2>
               <p className="text-xs text-content-muted mt-0.5">
-                Legt das Standard-Widget-Layout für alle Nutzer dieses Tenants fest. Nutzer können ihr eigenes Layout individuell anpassen.
+                Sets the default widget layout for all users of this tenant. Users can customize their own layout individually.
               </p>
             </div>
           </div>
           <div className="p-4 flex items-center gap-4">
             <div className="flex-1">
               <p className="text-sm text-content-secondary">
-                Tenant-Standard:{' '}
+                Tenant default:{' '}
                 <span className="font-medium text-content">
                   {tenantDefaultWidgets && tenantDefaultWidgets.length > 0
-                    ? `${tenantDefaultWidgets.length} Widget${tenantDefaultWidgets.length !== 1 ? 's' : ''} konfiguriert`
-                    : 'Noch kein Standard festgelegt (Systemvorgabe aktiv)'}
+                    ? `${tenantDefaultWidgets.length} Widget${tenantDefaultWidgets.length !== 1 ? 's' : ''} configured`
+                    : 'No default set yet (system default active)'}
                 </span>
               </p>
             </div>
@@ -970,7 +970,7 @@ export default function AdminPage() {
               className="btn-secondary text-sm flex items-center gap-2"
             >
               <LayoutDashboard size={14} />
-              Tenant-Standard-Dashboard bearbeiten
+              Edit Tenant Default Dashboard
             </button>
           </div>
         </div>