fix(maps): one imagery choice per map, not one for two of them

The world map and the grid-square map shared a single key, so picking
satellite imagery to look at grids repainted the main map too, and there
was no way to have terrain on one and streets on the other. They are
different maps answering different questions, and the imagery that suits
one is not the imagery that suits the next.

Four keys now, one per map, in lib/mapBase beside the remembered views —
named in one place so a rename cannot silently orphan somebody's choice —
and portable, so a copied data folder brings them along. The grid map
inherits whatever was set under the old shared key rather than being
reset to the default: an operator who chose imagery there keeps it.

frontend/src/components/MainMap.tsx

@@ -3,6 +3,7 @@ import L from 'leaflet';
 import 'leaflet/dist/leaflet.css';
 import { nightPolygon } from '../lib/greyline';
 import { gridToLatLon, gridSquareBounds, greatCirclePoints, pathBetween, destinationPoint } from '@/lib/maidenhead';
+import { loadMapBase, saveMapBase, MAP_BASE_WORLD } from '@/lib/mapBase';
 import { writeUiPref } from '@/lib/uiPref';
 import { formatDistance } from '@/lib/units';
 import { loadMapView, saveMapView, MAP_VIEW_WORLD } from '@/lib/mapView';
@@ -116,9 +117,10 @@ export const BASEMAPS: Record<BasemapKey, { label: string; url: string; attr: st
                attr: 'Tiles &copy; Esri — Source: Esri, Maxar, Earthstar Geographics',
                labelsUrl: 'https://server.arcgisonline.com/ArcGIS/rest/services/Reference/World_Boundaries_and_Places/MapServer/tile/{z}/{y}/{x}' },
 };
+// loadBasemap is the WORLD map's imagery. Each map keeps its own — see
+// lib/mapBase, which is where the keys live.
 export function loadBasemap(): BasemapKey {
-  const v = localStorage.getItem('opslog.mapBasemap');
-  return v === 'voyager' || v === 'street' || v === 'satellite' ? v : 'light';
+  return loadMapBase(MAP_BASE_WORLD, 'light');
 }
 
 // addBasemap (re)installs the imagery layer and, for satellite, its transparent
@@ -445,7 +447,7 @@ export function WorldMap({ fromGrid, toGrid, fromLabel, toLabel, beamAzimuths, b
           <button
             key={k}
             type="button"
-            onClick={() => { setBasemap(k); writeUiPref('opslog.mapBasemap', k); }}
+            onClick={() => { setBasemap(k); saveMapBase(MAP_BASE_WORLD, k); }}
             title={`Basemap: ${BASEMAPS[k].label}`}
             className={`px-2 py-1 text-[11px] font-medium transition-colors ${
               basemap === k ? 'bg-primary text-primary-foreground' : 'bg-card/90 text-muted-foreground hover:bg-card'