feat(find_model gui): toggle to overlay full SAM21 cluster (yellow)

Adds a 4th checkbox 'Punti SAM21 (gialli)' that draws the full SAM21
segmentation (input to both the plane partition and Open3DModel) on
the RGB result canvas. The backend now returns sam21_points_px and
n_sam21 alongside the existing accepted/excluded splits.

Useful as a sanity check: visualise what was actually segmented vs
what the plane filter kept/rejected vs where the STL got placed.

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

web/index.html

@@ -268,6 +268,7 @@
         <label class="inline"><input type="checkbox" id="findShowModel" checked> Punti modello (blu)</label>
         <label class="inline"><input type="checkbox" id="findShowAccepted"> Punti accettati (verdi)</label>
         <label class="inline"><input type="checkbox" id="findShowExcluded"> Punti esclusi (rossi)</label>
+        <label class="inline"><input type="checkbox" id="findShowSam21"> Punti SAM21 (gialli)</label>
       </div>
 
       <pre id="findDetails" class="log small">—</pre>