Rename to Avrundningsfel, add Öresutjämning row, round total

Summary now shows:
- Product rows
- Avrundningsfel (rounding error)
- Totalt (rounded to nearest integer)
- Öresutjämning (difference from transactions to rounded total)

templates/customer.html

@@ -165,6 +165,12 @@
                     <td>{{ product.amount }} Kr</td>
                 </tr>
                 {% endfor %}
+                <tr>
+                    <td>Avrundningsfel</td>
+                    <td></td>
+                    <td></td>
+                    <td>{{ customer.summary.avrundningsfel }} Kr</td>
+                </tr>
                 <tr class="grand-total-row">
                     <td>Totalt</td>
                     <td>{{ customer.summary.total_volume }}</td>