feat(map): toggle the trip map between default and satellite tiles

Add a corner button on the Leaflet trip map that switches the base layer
between the default street tiles and a tokenless satellite view (ESRI
World Imagery). The choice is a per-user setting (map_base_layer) that
persists and applies instantly, including offline. GL-renderer satellite
styles are unchanged.

Closes #993

client/src/types.ts

@@ -122,6 +122,8 @@ export interface Settings {
   map_always_show_routes?: boolean
   optimize_from_accommodation?: boolean
   map_provider?: 'leaflet' | 'mapbox-gl' | 'maplibre-gl'
+  /** Leaflet base layer: default street tiles or a satellite/aerial view. */
+  map_base_layer?: 'default' | 'satellite'
   mapbox_access_token?: string
   mapbox_style?: string
   maplibre_style?: string