feat(work_plane gui): show fit inliers as blue scatter in 3D, drop wrong plane mesh

The 0.4x0.4 PlaneGeometry square was a fixed-size proxy that did not
reflect the actual ROI extent — visually misleading. Replace it with
an explicit blue point cloud of the inliers fed to fit_plane_svd, so
the user can see *exactly* which depth pixels the plane was fit on.

- Backend: fit response now returns fit_points_camera (CV frame) instead
  of the (parallax-shifted) RGB pixel projection.
- Frontend: drop the purple plane mesh; new three.setFitInliers paints
  the inliers in blue. The RGB overlay no longer carries blue dots.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/presa_mano/api/work_plane_routes.py

@@ -11,7 +11,6 @@ from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 
 from ..perception import filter_outliers, nuwa_to_cv_frame
-from ..perception.projection import K_from_intrinsics, project_camera_to_pixels
 
 log = logging.getLogger(__name__)
 from ..perception.work_plane import (
@@ -113,32 +112,13 @@ def fit(req: FitReq) -> dict[str, Any]:
         idx = np.random.choice(len(scene_cv), req.max_preview_points, replace=False)
         scene_cv = scene_cv[idx]
 
-    # Project the inliers used for the fit back to RGB pixels so the GUI can
-    # overlay them on the live image (verify visually which depth pixels were
-    # actually fed to fit_plane_svd). NOTE: pointcloud_roi's docstring claims
-    # CV frame, but in practice the raw `pc` arrives in Nuwa native frame
-    # (Z<0 forward). After nuwa_to_cv_frame + outlier mask, `pts_cv` is the
-    # canonical CV-frame inlier set, so use it.
-    fit_points_px: list[list[int]] = []
-    try:
-        intr_full = nuwa.intrinsics()
-        rgb_intr = intr_full.get("rgb")
-        if rgb_intr and rgb_intr.get("fx") and rgb_intr.get("cx"):
-            # Inject width/height so K_from_intrinsics has all required keys.
-            W = int(rgb_intr.get("width", 640))
-            H = int(rgb_intr.get("height", 480))
-            rgb_intr = {**rgb_intr, "width": W, "height": H}
-            K = K_from_intrinsics(rgb_intr)
-            with sw.step("project_inliers"):
-                fit_points_px = project_camera_to_pixels(pts_cv, K, (W, H)).tolist()
-    except Exception as e:
-        log.warning("could not project fit inliers to RGB pixels: %s", e)
-
     return {
         "plane": asdict(wp),
         "preview": True,
         "points_camera": scene_cv.tolist(),
-        "fit_points_px": fit_points_px,
+        # 3D inliers actually used for fit_plane_svd, in CV camera frame.
+        # The GUI scatters these in blue inside the 3D viewer.
+        "fit_points_camera": pts_cv.tolist(),
         "timings_ms": sw.as_dict(),
     }