web UI: center the check mark (1px left/up)

src/web/ui.html

@@ -81,7 +81,7 @@
   .actions input[type=checkbox]:hover { border-color: var(--accent); }
   .actions input[type=checkbox]:checked { background: var(--accent); border-color: var(--accent); }
   .actions input[type=checkbox]:checked::after {
-    content: ""; position: absolute; left: 4.5px; top: 1px;
+    content: ""; position: absolute; left: 3.5px; top: 0;
     width: 4px; height: 8px; border: solid #fff; border-width: 0 2px 2px 0;
     transform: rotate(45deg);
   }