feat: show target_date in goal list next to target value

- Start value already showed start_date in parentheses
- Now target value also shows target_date in parentheses
- Consistent UX: both dates visible at their respective values

frontend/src/pages/GoalsPage.jsx

@@ -695,6 +695,11 @@ export default function GoalsPage() {
                                 <div>
                                   <span style={{ color: 'var(--text2)' }}>Ziel:</span>{' '}
                                   <strong style={{ color: catInfo.color }}>{goal.target_value} {goal.unit}</strong>
+                                  {goal.target_date && (
+                                    <span style={{ fontSize: 12, color: 'var(--text3)', marginLeft: 4 }}>
+                                      ({dayjs(goal.target_date).format('DD.MM.YY')})
+                                    </span>
+                                  )}
                                 </div>
                               </div>