Fix server hang: run map loading off the main thread

The preloaded-maps completion callback was wired by calling
ImageMapManager.loadMaps() directly on the main thread during onEnable.
loadMaps()'s synchronous prefix loads saved maps via
URLStaticImageMapLoader.load(), which blocks on future.get() for a
MapUtils.createMap() task scheduled onto the main thread -> the main
thread waits on itself and the server hangs while enabling ImageFrame.

Restore the Scheduler.runTaskAsynchronously wrapper so loadMaps runs off
the main thread (as upstream did), while keeping the completion callback
so preloaded maps are still created only after saved maps finish loading.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/src/main/java/com/loohp/imageframe/ImageFrame.java

@@ -300,7 +300,7 @@ public class ImageFrame extends JavaPlugin {
 
         ImageMapLoaders.init();
 
-        imageMapManager.loadMaps(ifPlayerManager, this::loadPreloadedMaps);
+        Scheduler.runTaskAsynchronously(this, () -> imageMapManager.loadMaps(ifPlayerManager, this::loadPreloadedMaps));
 
         getServer().getConsoleSender().sendMessage(ChatColor.GREEN + "[ImageFrame] ImageFrame has been Enabled!");
     }