map: mount without waiting on the 500-call window; legend collapsible, hidden under open layers list

The Live map was held behind useCalls(500), measured at ~9.2s to first map
paint on prod; it now mounts as soon as the saved view is known (nodes are
awaited only when there is no view). The legend painted over the expanded
Leaflet layers list (z-1001 vs the map's z-0 stacking context): it is now a
<details> that hides while the list is open and starts folded on short
viewports.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/app/globals.css

@@ -193,6 +193,13 @@ html:not(.dark) .border-indigo-800 { border-color: #c8ccf6 !important; }
   z-index: 0;
 }
 
+/* The legend floats above the map's stacking context, so it would cover the
+ * expanded layers list (both live in the right-hand column). Hide it while the
+ * list is open. */
+.relative:has(.leaflet-control-layers-expanded) > .drb-legend {
+  visibility: hidden;
+}
+
 /* "Dark" base layer (#136): the prod tile URL is bright OSM, so dark-theme
  * overlay cards sat on a bright, busy map. Inverting the raster keeps the
  * keyless OSM source (CARTO's dark_all CDN proved flaky, #117) while giving