Upgrade dependencies to 6.0.0-rc.3

- Bump userfrosting/framework and sprinkle-core to 6.0.0-rc.3
- Bump userfrosting/vite-php-twig to 1.2.0
- Add vite_css_preload() calls in stylesheets_site.html.twig
- Upgrade Node.js from 20 to 24 in docker/app/Dockerfile
- Upgrade Docker vue image from node:22-bullseye to node:24-bookworm
- Run npm update

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lcharette

app/templates/content/stylesheets_site.html.twig

@@ -1,6 +1,8 @@
 {# This file should contain the list of css to include site wide. It will be injected into the base template headers #}
 {# The `app` entry from Webpack config is loaded here #}
 {% block stylesheets_site %}
+    {{ vite_css_preload('main.ts') }}
+    {{ vite_css_preload('search.ts') }}
     {{ vite_css('main.ts') }}
     {{ vite_css('search.ts') }}
     {{ vite_preload('main.ts') }}