scan-flow: geolocation scan, location-aware SMS throttle (log sender), finder contact, sms_enabled toggle, branding — 19/19 phase-2 scenarios pass

frontend/internal/sms/geo.go

@@ -0,0 +1,16 @@
+package sms
+
+import "math"
+
+const earthRadiusM = 6371000.0
+
+// HaversineMeters returns the great-circle distance between two points
+// (lat/lng in decimal degrees) in metres.
+func HaversineMeters(lat1, lng1, lat2, lng2 float64) float64 {
+	rad := func(d float64) float64 { return d * math.Pi / 180 }
+	dLat := rad(lat2 - lat1)
+	dLng := rad(lng2 - lng1)
+	a := math.Sin(dLat/2)*math.Sin(dLat/2) +
+		math.Cos(rad(lat1))*math.Cos(rad(lat2))*math.Sin(dLng/2)*math.Sin(dLng/2)
+	return 2 * earthRadiusM * math.Asin(math.Sqrt(a))
+}