Set up React Flow and add a base Diagram component

[lornakelly]

Summary

Integrate React Flow into the project and create an initial Diagram component that can render a small, node/edge graph so we can visually confirm the diagram layer is wired correctly.

Goals

• Prove React Flow is correctly installed and working in our stack.
• Establish the baseline Diagram component + folder structure for future diagram work.
• Render a simple static graph (nodes + edges) as a visible smoke test.

Non-Goals

• No Typescript SDK parsing.
• No custom node/edge UX beyond basic rectangles/labels.
• No auto-layout.

Description

Add React Flow as a dependency and create the diagram rendering baseline in src/diagram/:

• Install and configure React Flow (and required CSS import).
• Create a small static graph sample