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/components/LiveView.tsx

@@ -560,7 +560,11 @@ export function LiveView() {
           {/* Degraded → grey the map and its incident rail (the rail lives inside
               MapView, so it is wrapped here rather than edited). */}
           <div className={`w-full h-full transition-[filter,opacity] ${health.degraded ? "grayscale opacity-60" : ""}`}>
-          {nodesLoading || callsLoading || !mapState ? (
+          {/* Mount the map as soon as its starting view is known. The 500-call
+              window took ~9s on prod and used to hold the whole map back;
+              calls and paths now stream in once it lands. Nodes are only
+              awaited when there's no saved view, since they pick the centre. */}
+          {!mapState || (!mapState.view && nodesLoading) ? (
             <div className="w-full h-full flex items-center justify-center text-ink-muted text-sm">Loading map…</div>
           ) : (
             <MapView