feat(scene): add selected mesh outline

[DrHepa]

Closes #162

Summary

• Adds a violet outline around the selected mesh in the Generate viewer.
• Keeps the existing Viewer3D.tsx selection state as the source of truth.
• Uses @react-three/postprocessing Selection, Select, EffectComposer, and Outline.
• Keeps the composer bounded with multisampling={0}, resolutionScale={0.5}, and xRay={false} to avoid conflicts with the viewer gizmo/HUD.

Changes

File	Change
src/areas/generate/components/Viewer3D.tsx	Adds selected mesh outline rendering using existing selection state and postprocessing components.

Validation

Manual validation completed:

• Clicking the mesh shows the violet outline.
• Clicking empty space clears the outline.
• Gizmo/XYZ axes remain visible.
• Viewer remains responsive.
• Outline works while orbiting/auto-rotating.
• No backend/store/API changes.

Validation media:

• outline-modly2.mp4

Notes

The composer is tuned to avoid conflict with Drei GizmoHelper/HUD:

• multisampling={0}
• resolutionScale={0.5}
• xRay={false}
• GizmoHelper renderPriority={2}

No package/dependency changes were required.