Decide on and add coverage for Graph.tsx node-selection behavior

[pinecone-groundskeeper]

Background

Part of the test-coverage work tracked in #38, which flagged that explorer/src/app/components/Graph.tsx node selection may need Playwright rather than the existing Vitest + jsdom setup, since the component renders via react-force-graph-2d/3d-force-graph on canvas/WebGL — surfaces jsdom does not implement.

Problem

Graph.tsx has zero automated coverage today. Before writing tests, someone needs to decide the right tool:

• jsdom + mocked force-graph library — mock react-force-graph-2d/3d-force-graph entirely and unit-test the selection callback wiring (onNodeClick, the useEffects syncing selectedData/selectedNode into local state) in isolation from the actual canvas rendering. Cheap, but doesn't exercise real rendering.
• Playwright (or similar browser-based runner) — exercise real canvas/WebGL rendering and click-to-select interaction end-to-end. More realistic, but is new test infrastructure/tooling for this repo, which is a maintainer call, not something to add unilaterally in a maintenance slice.

Scope

• Decide which approach fits this repo (or whether both are warranted: an isolated logic test now, browser testing later).
• Implement the chosen automated coverage for node-selection behavior in Graph.tsx.

Acceptance criteria

• Node-selection logic in Graph.tsx is exercised by an automated test that runs in CI without live credentials, or
• If new tooling (e.g. Playwright) is the right call, that decision is made explicitly (not guessed at) before implementation, since introducing a new test runner/browser dependency is a bigger footprint than a single test file.
  Related to Expand explorer test coverage (Graph, API routes, streaming) #38

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