fix: use vite-plugin-symfony Stimulus loader and wrap react_component in div

The @symfony/stimulus-bundle loader generates an empty controllers.js,
so Stimulus controllers from controllers.json (including ux-react) were
never registered. Switching to vite-plugin-symfony/stimulus/helpers
uses the virtual:symfony/controllers module that properly reads
controllers.json. Also wrap react_component() output in a <div> since
it only renders data-attributes, not a full HTML element.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/bootstrap.js

@@ -1,4 +1,4 @@
-import { startStimulusApp } from '@symfony/stimulus-bundle';
+import { startStimulusApp } from 'vite-plugin-symfony/stimulus/helpers';
 
 const app = startStimulusApp();
 
@@ -15,5 +15,5 @@ window.resolveReactComponent = (name) => {
             .map(k => k.replace('./react/controllers/', '').replace('.jsx', ''));
         throw new Error(`React controller "${name}" does not exist. Possible values: ${available.join(', ')}`);
     }
-    return module.default;
+    return module;
 };