feat(maps): remembered views for the FT and grid maps, and no wasted first paint

The world map was the only one that remembered anything. Panning and
zooming a map is the operator saying which part of the world they work,
and on the FT decodes map and the grid-square map that was thrown away on
every tab switch. Both now keep centre and zoom, through one shared
helper (lib/mapView) that the world map uses too, and the keys are
portable so a copied data folder brings the views with it.

The world map also waits for the station's square before it paints. It
drew the world at 0 degrees and then moved to the operator's longitude —
a screenful of Esri tiles fetched and discarded on every first run. It is
now built once, knowing where it is looking; a profile with no locator
gets the default view after two seconds rather than a blank panel.

frontend/src/lib/mapView.ts

@@ -0,0 +1,31 @@
+// Remembered map views — where each map was left, per map.
+//
+// Panning and zooming a map is an operator saying "this is the part of the world
+// I work". Throwing that away on every tab switch made the maps something to
+// set up again each time rather than something to glance at, and the world map
+// was the only one that remembered anything.
+//
+// One key per map, and portable (see lib/uiPref) like the world map's own: a
+// copied data folder brings the views with it.
+import { writeUiPref } from '@/lib/uiPref';
+
+export type MapView = { lat: number; lon: number; zoom: number };
+
+// The keys in use. Named here rather than typed at each call site so a rename
+// cannot silently orphan somebody's saved view.
+export const MAP_VIEW_WORLD = 'opslog.mapView';
+export const MAP_VIEW_FT = 'opslog.ftMapView';
+export const MAP_VIEW_GRIDS = 'opslog.gridMapView';
+
+export function loadMapView(key: string): MapView | null {
+  try {
+    const v = JSON.parse(localStorage.getItem(key) || 'null');
+    return v && typeof v.zoom === 'number' && typeof v.lat === 'number' && typeof v.lon === 'number' ? v : null;
+  } catch {
+    return null; // corrupt or unreadable → open where the map would by default
+  }
+}
+
+export function saveMapView(key: string, lat: number, lon: number, zoom: number): void {
+  writeUiPref(key, JSON.stringify({ lat, lon, zoom }));
+}