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/GridSquareMap.tsx

@@ -6,7 +6,8 @@ import { GridSquares } from '../../wailsjs/go/main/App';
 import { gridSquareBounds, gridToLatLon } from '@/lib/maidenhead';
 import { useI18n } from '@/lib/i18n';
 import { cn } from '@/lib/utils';
-import { BASEMAPS, addBasemap, loadBasemap, type BasemapKey } from '@/components/MainMap';
+import { BASEMAPS, addBasemap, type BasemapKey } from '@/components/MainMap';
+import { loadMapBase, saveMapBase, MAP_BASE_GRIDS, MAP_BASE_WORLD } from '@/lib/mapBase';
 import { writeUiPref } from '@/lib/uiPref';
 import { loadMapView, saveMapView, MAP_VIEW_GRIDS } from '@/lib/mapView';
 
@@ -86,7 +87,9 @@ export function GridSquareMap({ myGrid, className }: { myGrid?: string; classNam
     () => (SCOPES.some((s) => s.key === localStorage.getItem(SCOPE_KEY))
       ? (localStorage.getItem(SCOPE_KEY) as ScopeKey) : 'DIGI'));
 
-  const [basemap, setBasemap] = useState<BasemapKey>(loadBasemap);
+  // This map's own imagery. It shared the world map's key until they were
+  // separated, so a choice made back then is inherited rather than reset.
+  const [basemap, setBasemap] = useState<BasemapKey>(() => loadMapBase(MAP_BASE_GRIDS, 'light', MAP_BASE_WORLD));
   const [confColour, setConfColour] = useState(() => localStorage.getItem(COL_CONFIRMED_KEY) ?? '');
   const [workedColour, setWorkedColour] = useState(() => localStorage.getItem(COL_WORKED_KEY) ?? '');
   // Repaint the squares when the THEME changes, not the basemap: the fills come
@@ -253,7 +256,7 @@ export function GridSquareMap({ myGrid, className }: { myGrid?: string; classNam
         </span>
         <select
           value={basemap}
-          onChange={(e) => { const v = e.target.value as BasemapKey; setBasemap(v); writeUiPref('opslog.mapBasemap', v); }}
+          onChange={(e) => { const v = e.target.value as BasemapKey; setBasemap(v); saveMapBase(MAP_BASE_GRIDS, v); }}
           title={t('gsm.basemap')}
           className="h-6 rounded border border-border bg-background px-1 text-[11px]"
         >