Frontend - natural language querying in visualization designer

What does this MR do and why?

Adds UI for generating a product analytics CubeJS visualization via AI.

Refactors viz designer + child components to support setting state from the generated query (this could also be useful later if we add support for editing visualizations).

Improved UI and better error handling will follow in subsequent MRs.

Note that this experiment is behind a default off feature flag.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
	(FF disabled)
	(FF enabled)
	(Loading)
	(Generated query is applied)

How to set up and validate locally

Apply setup steps for backend MR this one depends on:

• Enable AI features locally: https://docs.gitlab.com/ee/development/ai_features/#test-ai-features-locally
• Make sure you've enabled Vertex access and configured it with your GDK. This action does not use Anthropic's API, only Vertex.
• Enable the :generate_cube_query flag locally. NEW

Then:

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Enter ai prompts in the new UI

Related to #435259 (closed)