add hover tooltip to rent expenses column

ui.js

@@ -214,6 +214,12 @@ const build_table = (result) => {
 
     const td_rent_cost = document.createElement('td');
     td_rent_cost.textContent = fmt_eur(result.cum_rent_arr[i]);
+    td_rent_cost.classList.add('has-tip');
+    td_rent_cost.addEventListener('mouseenter', (e) => show_tooltip(e, [
+      `<span style="color:#8b949e;font-size:0.72rem;text-transform:uppercase;letter-spacing:.05em">dieses Jahr</span>`,
+      tip_row('Kaltmiete',   d.yr_rent,        '#2dd4bf'),
+      tip_row('Versicherung', d.yr_renters_ins, '#2dd4bf'),
+    ].join('<br>')));
 
     tr.appendChild(document.createElement('td')).textContent = result.labels[i];
     tr.appendChild(td_buy_nw);