Add a zoom-level readout to the map control stack

Matches the small numbered square NPS keeps next to its own zoom
buttons — a passive display, not a button, so it's plain markup with
no click behavior.

public/css/style.css

@@ -472,6 +472,18 @@ html, body {
   word-break: break-word;
 }
 
+.zoom-level-ctrl__value {
+  width: 29px;
+  height: 29px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 12px;
+  font-weight: 600;
+  color: #333;
+  user-select: none;
+}
+
 .gpx-popup__time {
   color: #888;
   font-size: 12px;