Add map import and export with durable refresh progress

shared/map-refresh.ts

@@ -0,0 +1,17 @@
+export type MapCapture = { id: string; name: string; created_at: string };
+
+// A contribution can be newer than the original capture, even when both have
+// the same name/date in the picker. Keep deliberate choices; otherwise use the
+// newest eligible capture, including when accepting a contribution adds one.
+export function refreshCapture(sources: MapCapture[], chosen?: string) {
+  if (chosen && sources.some((source) => source.id === chosen)) return chosen;
+  return (
+    sources.reduce<MapCapture | undefined>(
+      (latest, source) =>
+        !latest || Date.parse(source.created_at) > Date.parse(latest.created_at)
+          ? source
+          : latest,
+      undefined,
+    )?.id ?? ""
+  );
+}