feat: limit players to one active exploration at a time

Server-side: check if any area is already in_progress before allowing
a new exploration to start. Client-side: derive hasActiveExploration
from the full areas list and disable all Explore buttons (with a
tooltip) whilst another area is underway. Closes #9.

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

@@ -38,6 +38,9 @@ export const ExplorationPanel = (): React.JSX.Element => {
 
   const zoneAreas = EXPLORATION_AREAS.filter((a) => a.zoneId === activeZoneId);
 
+  const hasActiveExploration =
+    explorationState?.areas.some((a) => a.status === "in_progress") ?? false;
+
   const handleStart = async (areaId: string): Promise<void> => {
     setPendingAreaId(areaId);
     try {
@@ -137,8 +140,9 @@ export const ExplorationPanel = (): React.JSX.Element => {
                 {status === "available" && (
                   <button
                     className="start-quest-button"
-                    disabled={isPending}
+                    disabled={isPending || hasActiveExploration}
                     onClick={() => { void handleStart(area.id); }}
+                    title={hasActiveExploration ? "An exploration is already in progress" : undefined}
                     type="button"
                   >
                     {isPending ? "Departing..." : `Explore (${formatDuration(area.durationSeconds)})`}