fix: correct checkbox_anzahl field name so Elektriker/Dübelarbeiten appear in email

The _anzahl suffix was placed outside the closing bracket in PHP array
notation (e.g. [e-herd]_anzahl), causing PHP to ignore it and overwrite
the checkbox value. Moved the suffix inside the bracket ([e-herd_anzahl])
so both checkbox and quantity values are stored as separate POST keys.

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

includes/class-form-renderer.php

@@ -564,7 +564,7 @@ class Umzugsliste_Form_Renderer {
 						<input type="checkbox" name="<?php echo esc_attr( $field_name ); ?>" value="ja">
 						<?php echo esc_html( $field['name'] ); ?>
 					</label>
-					<input type="text" name="<?php echo esc_attr( $field_name . '_anzahl' ); ?>" class="qty-small" placeholder="<?php echo esc_attr__( 'Qty.', 'siegel-umzugsliste' ); ?>">
+					<input type="text" name="<?php echo esc_attr( substr( $field_name, 0, -1 ) . '_anzahl]' ); ?>" class="qty-small" placeholder="<?php echo esc_attr__( 'Qty.', 'siegel-umzugsliste' ); ?>">
 				</div>
 				<?php
 				break;