ui: Show camera viewing ray when hovering a pixel in the image viewer

[behnamasadi]

Summary

Adds a rerun-style interaction to the model viewer: hovering a pixel in a
camera's image draws that pixel's back-projected viewing ray in the 3D view.

• Double-click a camera frustum → its photo opens in the image window (existing).
• Hover a pixel in that photo → a ray shoots from the camera center through the
  pixel into the 3D scene, updating live as the cursor moves and clearing when
  it leaves the image.
• If the hovered pixel is near an observed keypoint, the ray snaps to and
  highlights the corresponding 3D point (yellow); otherwise it extends across
  the scene along the pixel's direction.

This makes the 2D image and the 3D reconstruction directly cross-referenceable:
point at anything in a photo and see where it lies in 3D.

Demo

How it works

1. Capture the hovered pixel — the image window installs an event filter on
   its QGraphicsView viewport (mouse tracking on) and maps the cursor to image
   pixel coordinates.
2. Unproject to a world ray — Camera::CamFromImg(pixel) gives the normalized
   camera-frame direction (x, y, 1), transformed by the image pose
   (Inverse(image.CamFromWorld())) into a world-space ray from
   image.ProjectionCenter().
3. Snap to a 3D point — the nearest observed Point2D with a point3D_id
   within ~1% of the image size is found; if present, the ray ends exactly at
   that 3D point and highlights it.
4. Draw in 3D — a dedicated LinePainter (ray) and PointPainter (highlight)
   render in ModelViewerWidget::paintGL, using the same
   model_scale_ * (world + model_origin_) transform as the rest of the scene.

Notes

• Files: model_viewer_widget.{h,cc} (SetHoverRay/ClearHoverRay + painters),
  image_viewer_widget.{h,cc} (event filter + unprojection). image_viewer_widget.h
  moves graphics_scene_/graphics_view_ from private to protected so the
  database image viewer can attach the hover filter.
• GUI feature — built with -DGUI_ENABLED=ON and verified interactively; not
  covered by the headless unit tests. The ray geometry was checked numerically
  (computed pixel direction matches the camera→3D-point direction).

Note on the second commit (cherry-picked)

For convenience this branch also carries a second commit,
"ui: Add reprojection-error image colormap for camera frustums", cherry-picked
from #4579, so both GUI viewer features can be built and demoed from a single
branch. That colormap change is reviewed separately in #4579 — happy to drop
the commit here and keep this PR scoped to the hover-ray change alone if you
prefer.