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/app.js

@@ -540,7 +540,7 @@ $('findBtn').addEventListener('click', async () => {
   }
 });
 
-['findShowModel', 'findShowAccepted', 'findShowExcluded', 'findShowSam21'].forEach(id =>
+['findShowModel', 'findShowAccepted', 'findShowExcluded', 'findShowSam21', 'findShowPlane'].forEach(id =>
   $(id).addEventListener('change', drawFindResult));
 
 function drawFindResult() {
@@ -548,6 +548,21 @@ function drawFindResult() {
   const [W, H] = findResult.rgb_size;
   if (findCanvas.width !== W) { findCanvas.width = W; findCanvas.height = H; }
   findCtx.drawImage(findRgbImg, 0, 0);
+  // Plane polygon (background, semi-transparent fill + outline).
+  if ($('findShowPlane').checked) {
+    const poly = findResult.plane_polygon_px || [];
+    if (poly.length >= 3) {
+      findCtx.beginPath();
+      findCtx.moveTo(poly[0][0], poly[0][1]);
+      for (let i = 1; i < poly.length; i++) findCtx.lineTo(poly[i][0], poly[i][1]);
+      findCtx.closePath();
+      findCtx.fillStyle = 'rgba(155,109,255,0.20)';
+      findCtx.fill();
+      findCtx.strokeStyle = '#9b6dff';
+      findCtx.lineWidth = 2;
+      findCtx.stroke();
+    }
+  }
   const dot = (pts, color, r) => {
     findCtx.fillStyle = color;
     for (const [u, v] of pts) findCtx.fillRect(u - r, v - r, r*2+1, r*2+1);