show Restschuld as negative in buy net worth tooltip

calc.js

@@ -116,6 +116,7 @@ function run_simulation(p) {
       cum_rent_arr.push(Math.round(cum_rent_cost));
       detail_arr.push({
         house_value:     Math.round(home_value),
+        mortgage_balance: Math.round(balance),
         buyer_portfolio: Math.round(buyer_portfolio),
         yr_interest:     Math.round(yr_interest),
         yr_principal:    Math.round(yr_principal),

ui.js

@@ -193,6 +193,7 @@ const build_table = (result) => {
     td_buy_nw.addEventListener('mouseenter', (e) => show_tooltip(e, [
       tip_row('Immobilienwert', d.house_value, '#f59e0b'),
       tip_row('ETF-Portfolio',  d.buyer_portfolio, '#f59e0b'),
+      tip_row('Restschuld',    -d.mortgage_balance, '#ef4444'),
     ].join('<br>')));
 
     const td_rent_nw = document.createElement('td');