web: right-align the headers of number columns

"table.t th" sets text-align:left and is more specific than ".num", so
the headers of the counter columns on the dashboard and the statistics
page stayed left-aligned above right-aligned values.

html/index.html

@@ -89,6 +89,7 @@ table.t th{
   padding:6px 8px;border-bottom:1px solid var(--bd);white-space:nowrap;
 }
 table.t th[data-sort]{cursor:pointer;user-select:none}
+table.t th.num{text-align:right}
 table.t td{padding:6px 8px;border-bottom:1px solid var(--bd);vertical-align:middle}
 table.t tr:last-child td{border-bottom:0}
 table.t tr:hover td{background:var(--sel)}