feat(find_model gui): toggle to overlay work plane as a violet polygon

The find_model response now returns plane_polygon_px (4 corners of a
30 cm square anchored at the work plane centroid, projected to RGB
pixels via the same K used for the SAM21 cloud). The GUI gains a
'Piano (viola)' checkbox that draws the polygon as a translucent
violet quad with an outline, helping the operator see whether the
plane sits where expected relative to the object.

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

web/index.html

@@ -269,6 +269,7 @@
         <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>
+        <label class="inline"><input type="checkbox" id="findShowPlane"> Piano (viola)</label>
       </div>
 
       <pre id="findDetails" class="log small">—</pre>