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/api/products.ts

@@ -126,6 +126,8 @@ export interface ProductCadUploadResponse {
   file_hash: string
   status: string
   product_id: string
+  warnings?: string[]
+  existing_render_count?: number
 }
 
 export async function uploadProductCad(id: string, file: File): Promise<ProductCadUploadResponse> {

frontend/src/api/uploads.ts

@@ -13,6 +13,12 @@ export interface ExcelPreviewRow {
   has_step: boolean
   is_duplicate: boolean
   duplicate_of_row: number | null
+  step_conflict: boolean
+  step_conflict_existing_name: string | null
+  step_conflict_excel_name: string | null
+  cad_name_conflict: boolean
+  cad_name_conflict_other_name: string | null
+  cad_name_conflict_row: number | null
 }
 
 export interface ExcelPreviewResult {
@@ -26,6 +32,8 @@ export interface ExcelPreviewResult {
   has_step_count: number
   no_step_count: number
   duplicate_count: number
+  step_conflict_count: number
+  cad_name_conflict_count: number
   warnings: string[]
   rows: ExcelPreviewRow[]
   column_headers: string[]