Fix GizmoHelper ViewCube radius to use focus point

[raashish1601]

Summary

• Use the active focusPoint.current when computing the ViewCube camera radius.
• This keeps camera distance relative to the current orbit target instead of world origin when the target is offset.

Fixes #2374.

Validation

• npm exec -- eslint src/core/GizmoHelper.tsx
• npm exec -- tsc --noEmit --emitDeclarationOnly false --strict --jsx react (fails on existing unrelated errors in src/core/Splat.tsx and src/web/Select.tsx)

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