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/uiPref.ts

@@ -29,6 +29,9 @@ const PORTABLE_KEYS = [
   'opslog.bandMapBands',  // bands shown side-by-side in the Band Map tab
   'opslog.mapAutoZoomDX', // Main map: auto-zoom to the DX (vs free pan/zoom)
   'opslog.mapView',       // Main map: remembered free-pan view (lat/lon/zoom)
+  // The same, for the FT decodes map and the grid-square map: a view an
+  // operator set up is theirs, and it should follow the folder like the rest.
+  'opslog.ftMapView', 'opslog.gridMapView',
   'opslog.lookupOnBlur',  // run the callsign lookup on blur instead of while typing
   'opslog.groupDigitalSlots', // matrix + cluster: all digital modes count as ONE (DXCC-style) instead of per-mode slots
   'opslog.clusterShowFilters', // cluster filter sidebar shown (tab + Main pane)