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>

docker/app/Dockerfile

@@ -30,6 +30,14 @@ ENV APP_ENV=dev \
     POSTGRES_USER=app \
     POSTGRES_PASSWORD=pwd
 
+###
+# Composer install stage (provides vendor assets for node build)
+###
+FROM base AS composer-deps
+
+COPY composer.json composer.lock symfony.lock ./
+RUN composer install --no-dev --no-scripts --no-autoloader --prefer-dist
+
 ###
 # Node build stage (for prod assets)
 ###
@@ -37,6 +45,9 @@ FROM node:22-alpine AS node-build
 
 WORKDIR /app
 
+# Copy vendor UX assets so npm can resolve file: dependencies
+COPY --from=composer-deps /app/vendor/symfony/ux-react/assets vendor/symfony/ux-react/assets
+
 COPY package.json package-lock.json* ./
 RUN npm install