Draft: Integration for a Vue 3 island flow_builder

What does this MR do and why?

The MR introduces the Vue 3 integration point for the Flow Builder

This merge request introduces a new Vue 3 integration point that runs alongside the existing monolith. The changes create a separate, scoped Vue 3 application in a new vue3-apps folder with its own build system, then integrate it into the main application as a custom web component called <flow-builder>.

The integration is protected with the flow_builder feature flag.

The implementation includes all the necessary build configuration to compile the Vue 3 app into a JavaScript file that gets loaded into the main application. The existing chat interface remains unchanged and continues to work when the feature flag is disabled, making this a safe, gradual migration approach.

References

References #576901 and &19586

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flags for the global_ai_catalog and ai_catalog_flows (Product decisions) and flow_builder.
2. Run ./scripts/build_frontend_islands
3. Visit the Flow Creation Form
4. Scroll down to the Flow nodes

MR acceptance checklist

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