fix: date formatting for Dropbox data imports

src/App.tsx

@@ -240,6 +240,16 @@ const articleNoIdx = resolvedCols.ARTICLE_NO;
                  return val;
                }
 
+               // Handle date columns - Excel serial dates are numbers >= 25569 (Jan 1, 1970)
+               if (header.includes('date') || header.includes('launch') || header.includes('ready')) {
+                 if (typeof val === 'number' && val >= 25569 && val <= 60000) {
+                   const excelEpoch = new Date(1899, 11, 30);
+                   const date = new Date(excelEpoch.getTime() + val * 86400000);
+                   return date.toISOString().split('T')[0]; // Returns YYYY-MM-DD
+                 }
+                 return val;
+               }
+
                const formatKeywords = ['price', 'eur', 'cost', 'msrp', 'net', 'gross', 'netto', 'brutto', 'pp', 'pph', 'uvp', 'vpe', 'stk', 'nw', 'gw', 'weight', 'kg'];
                const shouldFormat = formatKeywords.some(kw => header.includes(kw));