[fix] themes: run \#invoke after class constructor (#5757)

Move \#invoke into a microtask that will run after current call stack completes

Closes: https://github.com/searxng/searxng/issues/5756

client/simple/src/js/Plugin.ts

@@ -23,7 +23,7 @@ export abstract class Plugin {
   protected constructor(id: string) {
     this.id = id;
 
-    void this.invoke();
+    queueMicrotask(() => this.invoke());
   }
 
   private async invoke(): Promise<void> {