FEATURE 2 - Mesh selection outline

[lightningpixel]

FEATURE 2 — Mesh selection outline

As a user viewing a generated mesh, I want a visible outline around the
mesh when I click it, so I get clear feedback that it's selected before acting
on it (transform, delete, export).

The selection state already exists in Viewer3D.tsx (selected,
onSelect, onPointerMissed, click on the <primitive>), but there is no
visual indication. This ticket adds only the outline rendering.

Scope

• Clicking the mesh draws a highlight outline around it.
• Clicking empty space (onPointerMissed) removes the outline.
• Outline follows the mesh across all view modes (solid / wireframe / normals
  / matcap / uv) and auto-rotate.
• Subtle, on-brand color (e.g. violet #8b5cf6, matching the app accent).

Implementation

• src/areas/generate/components/Viewer3D.tsx:
  • Wrap the scene contents in <Selection> and add <EffectComposer> with
    <Outline> (both from @react-three/postprocessing, already a dependency).
  • Wrap the mesh <primitive> in <Select enabled={selected}> so the outline
    is driven by the existing selected state — no new selection logic needed.
  • Tune <Outline> props: edgeStrength, visibleEdgeColor,
    hiddenEdgeColor, blur.
• No backend, store, or API changes.

Acceptance criteria

• Clicking the mesh shows an outline around it.
• Clicking empty space removes the outline.
• Outline renders correctly in every view mode and during auto-rotate.
• No noticeable performance regression (composer enabled only when needed).
• Existing Delete-to-remove and selection hint text still work.

Estimate: S

[DrHepa]

I’m going to take a first pass at this one.

I’ll keep it focused on the existing selection state in Viewer3D.tsx, using the already installed @react-three/postprocessing outline/selection components. No backend/store/API changes intended.