feat: persist exploration zone selection in sessionStorage

Applies the same sticky-zone pattern as the boss and quest panels.
The handleZoneSelect wrapper already existed — it just needed to write
to sessionStorage alongside updating state, and the useState initialiser
needed to read from sessionStorage on mount.

apps/web/src/components/game/explorationPanel.tsx

@@ -67,7 +67,9 @@ interface CollectResult {
 const ExplorationPanel = (): JSX.Element => {
   const { state, startExploration, collectExploration, formatNumber }
     = useGame();
-  const [ activeZoneId, setActiveZoneId ] = useState("verdant_vale");
+  const [ activeZoneId, setActiveZoneId ] = useState(() => {
+    return sessionStorage.getItem("elysium_explore_zone") ?? "verdant_vale";
+  });
   const [ pendingAreaId, setPendingAreaId ] = useState<string | null>(null);
   const [ lastResult, setLastResult ] = useState<CollectResult | null>(null);
 
@@ -116,6 +118,7 @@ const ExplorationPanel = (): JSX.Element => {
   function handleZoneSelect(id: string): void {
     setActiveZoneId(id);
     setLastResult(null);
+    sessionStorage.setItem("elysium_explore_zone", id);
   }
 
   const goldChange = lastResult?.response.event?.goldChange ?? 0;