Implement DATEV export functionality in DataViewer and enhance TransactionsTable with selection features and improved row styling. Update environment variables and add devServer configuration in webpack for better development experience.

client/src/components/cellRenderers/JtlRenderer.js

@@ -2,6 +2,30 @@ import React from 'react';
 
 const JtlRenderer = (params) => {
   const hasJTL = params.value;
+  
+  // Handle undefined state (database unavailable)
+  if (hasJTL === undefined) {
+    return (
+      <div style={{ display: 'flex', justifyContent: 'center', alignItems: 'center', height: '100%' }}>
+        <div style={{ 
+          width: '12px', 
+          height: '12px', 
+          borderRadius: '50%', 
+          backgroundColor: '#fff3cd', 
+          border: '1px solid #ffc107', 
+          display: 'flex', 
+          alignItems: 'center', 
+          justifyContent: 'center',
+          fontSize: '8px',
+          color: '#856404',
+          fontWeight: 'bold'
+        }}>
+          ?
+        </div>
+      </div>
+    );
+  }
+  
   const backgroundColor = hasJTL ? '#388e3c' : '#f5f5f5';
   const border = hasJTL ? 'none' : '1px solid #ccc';