ui: add graph renderer switch contract before WebGL

[msaad00]

Goal

Create a tested renderer decision point before adding Sigma/WebGL.

Scope

• Centralize selection between React Flow, existing 2D large-graph overview, and future WebGL renderer.
• Keep investigation behavior independent from renderer choice.

Acceptance

• One tested GraphRendererSwitch decision path.
• Existing React Flow and large-graph overview behavior is unchanged.
• Future Sigma renderer can plug in without reshaping graph pages.

Verification

• cd ui && npm test -- --run tests/large-graph-overview.test.ts
• cd ui && npm run typecheck