Fix book screens opening on server thread

src/main/java/net/dnorberg/rivencraft/client/ClientBookInteractionHandler.java

@@ -12,6 +12,12 @@ public final class ClientBookInteractionHandler {
     }
 
     public static void onRightClickItem(PlayerInteractEvent.RightClickItem event) {
+        // NeoForge posts this common event on both logical sides in an
+        // integrated game. Screens may only be created by the client copy.
+        if (!event.getEntity().level().isClientSide()) {
+            return;
+        }
+
         boolean linkingBook = event.getItemStack().is(ModItems.LINKING_BOOK.get());
         boolean descriptiveBook = event.getItemStack().is(ModItems.DESCRIPTIVE_BOOK.get());
         if (!linkingBook && !descriptiveBook) {