FEATURE 1 - Interactive transform gizmo

[lightningpixel]

FEATURE 1 — Interactive transform gizmo (move / rotate / scale in scene)

As a user viewing a generated mesh, I want to grab a gizmo and move,
rotate, or scale the mesh directly in the 3D scene, so I can position it
visually without typing numeric values or leaving the app.

Complements the numeric POST /optimize/transform ticket — this is the
interactive counterpart (previously marked "Tier 3 / out of scope").

Scope

• Gizmo attached to the loaded mesh when it is selected (selection state
  already exists in Viewer3D.tsx).
• Three modes: Translate / Rotate / Scale, switchable via toolbar buttons
  and keyboard (W / E / R).
• Gizmo dragging disables OrbitControls (drei handles this automatically when
  OrbitControls has makeDefault).
• Optional snapping (grid / 15° / 0.1) while holding a modifier key.
• "Apply" bakes the gizmo transform into the mesh so it persists to export;
  "Reset" reverts to the auto-centered pose.

Implementation

• Viewer src/areas/generate/components/Viewer3D.tsx:
  • Add <TransformControls> from @react-three/drei, attached to the mesh
    <primitive object={scene}> via a ref (lift a ref out of SceneMeshModel).
  • Render only when selected. Mode driven by new state gizmoMode: 'translate' | 'rotate' | 'scale' | null.
  • Reconcile with the existing auto-center useEffect (Viewer3D.tsx:201) so a
    user transform isn't overwritten on re-render (only auto-center on first
    load / model change).
• Toolbar ViewerToolbar.tsx: add a Move/Rotate/Scale button group
  (mirrors the existing MODES button pattern), plus gizmoMode props.
• Keyboard: extend the existing keydown handler (Viewer3D.tsx:396) with
  W/E/R to switch mode and Esc to exit gizmo (skip when an input is focused).
• Bake/persist (Apply): read the mesh's world matrix and send it to the
  backend to bake into the GLB. Reuse / extend POST /optimize/transform
  (api/routers/optimize.py) by accepting a raw 4×4 matrix, applying it at
  scene level with Scene.apply_transform() (preserves materials/UV), then
  updateCurrentJob + pushMeshUrl to reload the viewer on the result.

Acceptance criteria

• Selecting the mesh shows a gizmo; deselecting hides it.
• Translate / rotate / scale all work and update the mesh live.
• W/E/R switch modes; toolbar buttons reflect the active mode.
• OrbitControls is disabled while dragging the gizmo, re-enabled after.
• "Apply" bakes the transform into the GLB; re-loading keeps the new pose.
• "Reset" restores the auto-centered position.
• No interference with viewMode switching, screenshot, or Delete-to-remove.

Estimate: M

[iammojogo-sudo]

I'll work on this one