FEATURE 4 - Add point lights to the scene

[lightningpixel]

FEATURE 4 — Add point lights to the scene

As a user lighting a mesh in the viewer, I want to add point lights from
the Lighting panel, tweak their color/intensity, and move them around in the
scene, so I can light the model exactly how I want for preview and screenshots.

Context

Lighting today is two fixed lights (Sun + Fill) defined by LightSettings
(GeneratePage.tsx:130), edited in the LightPopover (GeneratePage.tsx:144)
and rendered in Viewer3D.tsx:448. This adds user-placed point lights into
that same panel.

Scope

• In the existing Lighting panel (LightPopover):
  • An "Add point" button that adds a new point light.
  • Below it, a small list of the added points, each row letting the user
    pick its color and intensity (and remove it).
• Each point light is shown in the 3D scene as a billboard icon — a
  recognizable light/bulb icon, rendered in world space at the light's
  position and always facing the camera (editor-style gizmo, like Blender/Unity
  light icons). It is not a plain sphere or dot.
• Point lights can be selected (click the icon) and moved in the scene
  via the transform gizmo.
• They render live alongside the existing Sun + Fill lights.

Implementation

• State (GeneratePage.tsx): add a pointLights: PointLight[] array next
  to LightSettings. Each entry: { id, position:[x,y,z], color, intensity }.
• UI (LightPopover, GeneratePage.tsx:144): add the "Add point" button
  and render one row per point (color picker + intensity slider + remove),
  mirroring the existing lightRow pattern.
• Viewer (Viewer3D.tsx):
  • Render the pointLights array as <pointLight> elements (passed down like
    lightSettings).
  • At each light's world position, render a <sprite> with a light/bulb
    icon texture as the in-scene marker — billboard, camera-facing. Reuse the
    sprite approach already used by GizmoBubbles (Viewer3D.tsx); the icon
    can be drawn to a canvas (like makeAxisLabelTexture) or loaded from an
    asset. Tint the icon with the light's color so it's identifiable.
  • Keep the icon at a readable on-screen size regardless of zoom.
• Selection + move: clicking a light's icon selects it; attach the
  transform gizmo (translate mode) to move it. Reuse the selection + gizmo
  mechanism from the interactive-transform ticket so light markers and the mesh
  share one selection model.
• No backend changes.

Acceptance criteria

• "Add point" adds a light; it appears in the scene as a billboard light
  icon (not a plain dot/sphere), always facing the camera.
• Each added point shows in the list with a color picker and intensity
  control; edits update both the light and its icon live.
• A point can be removed from the list (and its icon disappears).
• Clicking a light's icon selects it; the gizmo lets the user move it, and
  the light follows.
• Multiple point lights can coexist.
• Existing Sun + Fill lighting still works unchanged.

Notes

• Cap the number of point lights (e.g. 4–6) for realtime performance.
• Light-icon selection should not conflict with mesh selection — clicking a
  light selects the light, clicking the mesh selects the mesh.
• The light icon should not be affected by scene lighting (use
  toneMapped={false} like the existing gizmo sprites) so it stays legible.

Estimate: M