Keep flights-map arcs visible when zoom tier layers are removed

syncPolylines filtered endpoints by map.hasLayer(marker), which drops
polylines whose endpoint's zoom-tier layer was just removed by
syncVisibility. Result: in spider mode, arcs to small/distant cities
vanished when the user zoomed out.

Arc coordinates are fixed by the city's lat/lng; render them as long as
the markers exist in the index. User-level filtering (domestic /
international) already happens upstream in filterRoutes, so the
hasLayer gate was both incorrect and redundant.

src/features/flights-map/components/MapCanvas.tsx

@@ -258,20 +258,26 @@ export const MapCanvas: FC<MapCanvasProps> = ({
 
     layer.clearLayers();
 
+    // Arcs draw against the city's fixed coordinates, so we only need the
+    // marker to exist in the index. Gating on `map.hasLayer(m)` conflates two
+    // separate concerns: the zoom-tier LOD layers (which get added/removed as
+    // the user zooms) and the user's domestic/international filter. Checking
+    // `hasLayer` here made spider-mode destination arcs vanish when zooming
+    // out — the endpoint marker's tier layer had just been removed.
     for (const pl of polylines) {
       const styleOpts = POLYLINE_STYLES[pl.style];
 
-      const visible: L.Marker[] = [];
+      const resolved: L.Marker[] = [];
       for (const cityId of pl.cityIds) {
         const m = markerIndexRef.current.get(cityId);
-        if (m && map.hasLayer(m)) visible.push(m);
+        if (m) resolved.push(m);
       }
-      if (visible.length < 2) continue;
+      if (resolved.length < 2) continue;
 
       const segments: L.LatLng[] = [];
-      for (let i = 0; i < visible.length - 1; i++) {
-        const from = visible[i]!.getLatLng();
-        const to = visible[i + 1]!.getLatLng();
+      for (let i = 0; i < resolved.length - 1; i++) {
+        const from = resolved[i]!.getLatLng();
+        const to = resolved[i + 1]!.getLatLng();
         const arc = buildGreatCircleArc(from, to);
         segments.push(...(i === 0 ? arc : arc.slice(1)));
       }