feat: duplicate product detection — STEP conflict warnings on Excel import and CAD upload

- Excel preview detects when a product already has a different STEP file linked
- Excel preview detects intra-Excel conflicts (same product, different CAD model names)
- Product STEP upload warns when replacing an existing file and shows render count
- All warnings are non-blocking (amber badges, toast warnings)
- LEARNINGS.md: all open items resolved

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/Upload.tsx

@@ -4,7 +4,7 @@ import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { useNavigate } from 'react-router-dom'
 import {
   FileSpreadsheet, CheckCircle, X, Plus,
-  Info, PackagePlus, PackageCheck, Ban, FileBox, ArrowRight, Copy,
+  Info, PackagePlus, PackageCheck, Ban, FileBox, ArrowRight, Copy, AlertTriangle,
 } from 'lucide-react'
 import { toast } from 'sonner'
 import { uploadExcel, finalizeExcelImport, getImportValidation } from '../api/uploads'
@@ -335,6 +335,20 @@ export default function UploadPage() {
                 description="Same Product Series appears multiple times. Pre-unchecked — only first occurrence imported."
                 color="bg-status-warning-bg border-border-default text-status-warning-text"
               />
+              <StatCard
+                icon={<AlertTriangle size={18} className="text-amber-600" />}
+                value={previewResult.step_conflict_count ?? 0}
+                label="STEP conflicts"
+                description="Product exists with a different STEP file than referenced in Excel."
+                color="bg-amber-50 border-amber-200 text-amber-800"
+              />
+              <StatCard
+                icon={<AlertTriangle size={18} className="text-amber-600" />}
+                value={previewResult.cad_name_conflict_count ?? 0}
+                label="CAD name conflicts"
+                description="Same product appears in multiple rows with different CAD model names."
+                color="bg-amber-50 border-amber-200 text-amber-800"
+              />
             </div>
           </div>
 
@@ -459,11 +473,23 @@ export default function UploadPage() {
                         )}
                       </td>
                       <td className="px-4 py-2 text-center">
-                        {!hasId ? null : row.has_step ? (
-                          <CheckCircle size={14} className="text-green-500 mx-auto" aria-label="STEP file linked" />
-                        ) : (
-                          <X size={14} className="text-red-400 mx-auto" aria-label="No STEP file" />
-                        )}
+                        <div className="flex items-center justify-center gap-1">
+                          {!hasId ? null : row.has_step ? (
+                            <CheckCircle size={14} className="text-green-500" aria-label="STEP file linked" />
+                          ) : (
+                            <X size={14} className="text-red-400" aria-label="No STEP file" />
+                          )}
+                          {row.step_conflict && (
+                            <span title={`STEP conflict: DB has "${row.step_conflict_existing_name}", Excel references "${row.step_conflict_excel_name}"`}>
+                              <AlertTriangle size={14} className="text-amber-500" />
+                            </span>
+                          )}
+                          {row.cad_name_conflict && (
+                            <span title={`CAD name conflict with row ${row.cad_name_conflict_row}: "${row.cad_name_conflict_other_name}"`}>
+                              <AlertTriangle size={14} className="text-amber-500" />
+                            </span>
+                          )}
+                        </div>
                       </td>
                     </tr>
                   )