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/SummaryHeader.js

@@ -95,8 +95,7 @@ class SummaryHeader extends Component {
       selectedMonth, 
       summary, 
       loading, 
-      onMonthChange, 
-      onDownloadDatev 
+      onMonthChange 
     } = this.props;
 
     if (!summary) return null;
@@ -249,23 +248,19 @@ class SummaryHeader extends Component {
           <Grid item xs={6} sm={4} md={1}>
             <Box textAlign="center">
               <Typography variant="caption" color="textSecondary">JTL ✓</Typography>
-              <Typography variant="h6" sx={{ fontWeight: 'bold', color: '#388e3c', fontSize: { xs: '0.9rem', sm: '1.25rem' } }}>
-                {summary.jtlMatches || 0}
+              <Typography 
+                variant="h6" 
+                sx={{ 
+                  fontWeight: 'bold', 
+                  color: summary.jtlMatches === undefined ? '#856404' : '#388e3c', 
+                  fontSize: { xs: '0.9rem', sm: '1.25rem' } 
+                }}
+              >
+                {summary.jtlMatches === undefined ? '?' : summary.jtlMatches}
               </Typography>
             </Box>
           </Grid>
-          <Grid item xs={12} sm={4} md={2}>
-            <Button
-              variant="contained"
-              startIcon={<DownloadIcon />}
-              onClick={onDownloadDatev}
-              disabled={!selectedMonth || loading}
-              size="small"
-              sx={{ height: 'fit-content' }}
-            >
-              DATEV Export
-            </Button>
-          </Grid>
+
         </Grid>
       </Paper>
     );