feat: add visual Graph Builder tool to Burr UI

[skrawcz]

This takes work from @jaeyow and #572.

Adds a drag-and-drop graph editor for designing Burr application graphs visually and exporting as Python code or JSON.

Changes

• these are UI only changes to add new graph-builder

Key changes:

• New /graph-builder route with full visual editor (ReactFlow v12)
• Migrate existing GraphView from reactflow v11 to @xyflow/react v12
• Remove reactflow and @tisoap/react-flow-smart-edge dependencies
• Per-node async/streaming toggles matching Burr's 4 action variants
• Python code generation with correct decorators and signatures
• 3 pre-built example graphs (MultiModal Chatbot, CRAG, Streaming)
• localStorage auto-save/restore of graph state
• Empty-state onboarding overlay and structured help sidebar
• Fix appcontainer layout for full-height content

How I tested this

• locally

Notes

Next would be to unify the color schemes with the observability components.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[github-actions]

A preview of b71f715 is uploaded and can be seen here:

✨ https://burr.dagworks.io/pull/667 ✨

Changes may take a few minutes to propagate. Since this is a preview of production, content with draft: true will not be rendered. The source is here: https://github.com/apache/burr/tree/gh-pages/pull/667/