[FIX] app: validate props for root component in dev mode

Before this commit, only sub components were validated.

src/app/app.ts

@@ -60,6 +60,9 @@ export class App<
 
   mount(target: HTMLElement, options?: MountOptions): Promise<Component<P, E> & InstanceType<T>> {
     App.validateTarget(target);
+    if (this.dev) {
+      this.helpers.validateProps(this.Root, this.props);
+    }
     const node = this.makeNode(this.Root, this.props);
     const prom = this.mountNode(node, target, options);
     this.root = node;