Add a toggleable IATA border layer to the map

Draw each active IATA's GeoJSON region border as an outline over a faint
fill beneath the node markers, behind a Map Settings toggle that's off by
default and shareable via a URL param. IATAs with no border answer 204
and are skipped.

tests/features/map/useMapBordersData.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect } from "vitest";
+import { mergeBorders } from "../../../src/features/map/useMapBordersData";
+import type { Feature, Polygon } from "geojson";
+
+const poly = (id: number): Feature<Polygon> => ({
+  type: "Feature",
+  properties: { name: `p${id}` },
+  geometry: { type: "Polygon", coordinates: [[[id, 0], [id + 1, 0], [id + 1, 1], [id, 0]]] },
+});
+
+describe("mergeBorders", () => {
+  it("drops IATAs with no border and stamps the iata onto each feature's properties", () => {
+    const fc = mergeBorders([
+      { iata: "YOW", border: poly(0) },
+      { iata: "YYZ", border: null },
+      { iata: "YUL", border: poly(5) },
+    ]);
+
+    expect(fc.type).toBe("FeatureCollection");
+    expect(fc.features).toHaveLength(2);
+    expect(fc.features.map((f) => f.properties.iata)).toEqual(["YOW", "YUL"]);
+    // existing properties and geometry survive the merge
+    expect(fc.features[0]!.properties.name).toBe("p0");
+    expect(fc.features[0]!.geometry).toEqual(poly(0).geometry);
+  });
+
+  it("returns an empty FeatureCollection when nothing has a border", () => {
+    const fc = mergeBorders([{ iata: "YOW", border: null }]);
+    expect(fc.features).toHaveLength(0);
+  });
+});