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>

web/index.html

@@ -192,7 +192,7 @@
       <div class="canvas-wrap" style="width:min(100%,720px)">
         <canvas id="wp3dCanvas" width="720" height="360" style="display:block;background:#0a0d11;border:1px solid var(--border);border-radius:4px"></canvas>
       </div>
-      <div class="marker-info">vista camera · piano viola · ciano = punti tenuti · rosso = esclusi (sotto piano−offset) · drag = ruota · wheel = zoom</div>
+      <div class="marker-info">vista camera · blu = punti usati per il fit · ciano = scena tenuta · rosso = scena esclusa (sotto piano−offset) · drag = ruota · wheel = zoom</div>
     </div>
     <div>
       <h2>Calibrazione</h2>