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/components/FTMapPanel.tsx

@@ -5,6 +5,7 @@ import { gridToLatLon, greatCirclePoints, splitAtAntimeridian } from '@/lib/maid
 import { BASEMAPS, type BasemapKey } from '@/components/MainMap';
 import { cn } from '@/lib/utils';
 import { useI18n } from '@/lib/i18n';
+import { loadMapView, saveMapView, MAP_VIEW_FT } from '@/lib/mapView';
 
 // FT Map — the live decode feed as geography: every station decoded in the
 // last half hour, an arc from the operator's own square to theirs, coloured by
@@ -39,6 +40,10 @@ const MAX_ARCS = 300;
 const MAX_AGE_MS = 30 * 60_000;
 
 export function FTMapPanel({ decodes, myGrid }: { decodes: FTMapDecode[]; myGrid: string }) {
+  // Where this map was left. Panning and zooming it is the operator saying which
+  // part of the world they are working; throwing that away on every tab switch
+  // made it something to set up again rather than something to glance at.
+  const saved = useRef(loadMapView(MAP_VIEW_FT));
   const { t } = useI18n();
   const divRef = useRef<HTMLDivElement>(null);
   const mapRef = useRef<L.Map | null>(null);
@@ -58,7 +63,15 @@ export function FTMapPanel({ decodes, myGrid }: { decodes: FTMapDecode[]; myGrid
       // No maxBounds: with the world smaller than the window the clamp
       // dragged every zoom into a corner. noWrap tiles alone keep one world.
       worldCopyJump: false, preferCanvas: true,
-      center: [25, 0], zoom: 2, minZoom: 2,
+      center: saved.current ? [saved.current.lat, saved.current.lon] : [25, 0],
+      zoom: saved.current ? saved.current.zoom : 2,
+      minZoom: 2,
+    });
+    // Every move, not just the deliberate ones: a zoom is as much a choice as a
+    // pan, and there is no moment afterwards at which to ask.
+    m.on('moveend', () => {
+      const c = m.getCenter();
+      saveMapView(MAP_VIEW_FT, c.lat, c.lng, m.getZoom());
     });
     mapRef.current = m;
     layerRef.current = L.layerGroup().addTo(m);