Add a distance/area measuring tool

A ruler button in the top-left control stack, matching NPS's own
Area/Distance ruler. Click to add vertices; distance sums haversine
segment lengths, area uses the spherical-excess polygon formula
(same one turf.js's area() uses) rather than a flat shoelace, since a
flat approximation drifts at park-boundary scale. SI units only
(m/km, m²/km²) — no imperial toggle.

public/css/style.css

@@ -472,6 +472,82 @@ html, body {
   word-break: break-word;
 }
 
+.measure-ctrl {
+  position: relative;
+}
+
+.measure-ctrl__button svg {
+  display: block;
+  margin: 0 auto;
+  width: 18px;
+  height: 18px;
+}
+
+.measure-ctrl__button.active {
+  background: #dbe9ff;
+}
+
+.measure-panel {
+  position: absolute;
+  top: 0;
+  left: 100%;
+  margin-left: 8px;
+  z-index: 20;
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  min-width: max-content;
+  background: #fff;
+  border: 1px solid #ddd;
+  border-radius: 6px;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+  padding: 6px 8px;
+  font-size: 13px;
+}
+
+.measure-panel[hidden] {
+  display: none;
+}
+
+.measure-panel__mode {
+  padding: 4px 8px;
+  border: none;
+  border-radius: 4px;
+  background: none;
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+}
+
+.measure-panel__mode:hover {
+  background: #f0f0f0;
+}
+
+.measure-panel__mode.active {
+  background: #dbe9ff;
+  font-weight: 600;
+}
+
+.measure-panel__result {
+  min-width: 4em;
+  color: #333;
+  font-variant-numeric: tabular-nums;
+}
+
+.measure-panel__clear {
+  border: none;
+  background: none;
+  font-size: 16px;
+  line-height: 1;
+  color: #888;
+  cursor: pointer;
+  padding: 0 2px;
+}
+
+.measure-panel__clear:hover {
+  color: #333;
+}
+
 .zoom-level-ctrl__value {
   width: 29px;
   height: 29px;