fix: preserve newlines in CSV export for readable multi-line fields

Newlines within quoted CSV cells render correctly in Excel and Google
Sheets. Previously all newlines were collapsed to spaces, making
bullet lists and paragraphs unreadable.

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

backend/src/services/csvExportService.ts

@@ -215,15 +215,16 @@ class CSVExportService {
    */
   private static escapeCSVValue(value: string): string {
     if (!value) return '';
-    
-    // Replace newlines with spaces and trim
-    const cleanValue = value.replace(/\n/g, ' ').replace(/\r/g, ' ').trim();
-    
+
+    // Normalize line endings but preserve newlines for readability in spreadsheet cells
+    const cleanValue = value.replace(/\r\n/g, '\n').replace(/\r/g, '\n').trim();
+
     // If value contains comma, quote, or newline, wrap in quotes and escape internal quotes
+    // Excel/Google Sheets render newlines within quoted cells correctly
     if (cleanValue.includes(',') || cleanValue.includes('"') || cleanValue.includes('\n')) {
       return `"${cleanValue.replace(/"/g, '""')}"`;
     }
-    
+
     return cleanValue;
   }