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/controllers.json

@@ -1,5 +1,11 @@
 {
     "controllers": {
+        "@symfony/ux-react": {
+            "react": {
+                "enabled": true,
+                "fetch": "eager"
+            }
+        },
         "@symfony/ux-turbo": {
             "turbo-core": {
                 "enabled": true,
@@ -9,12 +15,6 @@
                 "enabled": false,
                 "fetch": "eager"
             }
-        },
-        "@symfony/ux-react": {
-            "react": {
-                "enabled": true,
-                "fetch": "eager"
-            }
         }
     },
     "entrypoints": []