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/lib/mapBase.ts

@@ -0,0 +1,38 @@
+// Which imagery each map draws on — one choice per map.
+//
+// The world map and the grid-square map used to share a single key, so picking
+// satellite imagery to look at grids also repainted the main map, and there was
+// no way to have terrain on one and plain 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.
+//
+// Portable (see lib/uiPref) like the remembered views in lib/mapView: a copied
+// data folder brings the choices with it.
+import { writeUiPref } from '@/lib/uiPref';
+import type { BasemapKey } from '@/components/MainMap';
+
+// The keys in use. Named here rather than typed at each call site so a rename
+// cannot silently orphan somebody's choice.
+export const MAP_BASE_WORLD = 'opslog.mapBasemap';
+export const MAP_BASE_GRIDS = 'opslog.gridMapBase';
+export const MAP_BASE_FT = 'opslog.ftmapBase';
+export const MAP_BASE_SAT = 'opslog.satMapBase';
+
+const VALID = ['light', 'voyager', 'street', 'satellite'];
+
+// loadMapBase reads one map's choice.
+//
+// inheritFrom exists for the split: the grid map's choice lived under the world
+// map's key until they were separated, so an operator who had chosen imagery
+// there keeps it instead of being silently reset to the default.
+export function loadMapBase(key: string, fallback: BasemapKey, inheritFrom?: string): BasemapKey {
+  const read = (k: string) => {
+    const v = localStorage.getItem(k);
+    return v && VALID.includes(v) ? (v as BasemapKey) : null;
+  };
+  return read(key) ?? (inheritFrom ? read(inheritFrom) : null) ?? fallback;
+}
+
+export function saveMapBase(key: string, v: BasemapKey): void {
+  writeUiPref(key, v);
+}