Initial release: collaborative trip planner

Multi-user trip planning web app in a single Docker container. Mullvad-style token accounts, trip sharing via join codes, day-by-day calendar with typed entries (activity, hotel, travel, flight, rental car, immigration, note), multi-leg flight segments with bundled IATA airport dataset, Leaflet/OSM map with per-leg great-circle km (air vs ground), rough km-driven vs rental included-km comparison, cost splitting with settle-up suggestions, flip-clock departure countdown. Node 20 + Express + SQLite (WAL, additive migrations), vanilla JS SPA, 44 API tests.

src/server/util/distance.js

@@ -0,0 +1,17 @@
+// Great-circle distance between two lat/lng points, in kilometers.
+
+const EARTH_RADIUS_KM = 6371;
+
+function toRad(deg) {
+  return (deg * Math.PI) / 180;
+}
+
+export function haversineKm(lat1, lng1, lat2, lng2) {
+  const dLat = toRad(lat2 - lat1);
+  const dLng = toRad(lng2 - lng1);
+  const a =
+    Math.sin(dLat / 2) ** 2 +
+    Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLng / 2) ** 2;
+  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
+  return EARTH_RADIUS_KM * c;
+}