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/js/icons.js

@@ -26,6 +26,12 @@ export const GNSS_ICON = `
   <path d="M12 2v3M12 19v3M2 12h3M19 12h3"/>
 </svg>`;
 
+export const RULER_ICON = `
+<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linejoin="round" stroke-linecap="round">
+  <path d="M4 15 15 4l5 5-11 11-5-5z"/>
+  <path d="M9 10l1.5 1.5M12 7l1.5 1.5M15 4l1.5 1.5M6 13l1.5 1.5"/>
+</svg>`;
+
 export const NEW_FOLDER_ICON = `
 <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round">
   <path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V7z"/>