fix(work_plane gui): preserve ROI overlay across RGB live-refresh

Setting canvas.width/height clears its bitmap; the live RGB tick was
firing the img.load handler every second, wiping the ROI rectangle and
breaking the two-click flow (third click was interpreted as restart).

- Only mutate canvas size when image resolution actually changes.
- Re-paint the overlay after each load so the ROI persists.

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

web/work_plane.js

@@ -61,12 +61,17 @@ function startRgbLoop() {
   }
 
   function syncOverlaySize() {
-    overlay.width = img.naturalWidth || 640;
-    overlay.height = img.naturalHeight || 480;
+    // Setting width/height on a canvas wipes its content. Only re-set when
+    // the image resolution actually changes (rare) so the ROI overlay survives
+    // each RGB live-refresh tick.
+    const w = img.naturalWidth || 640;
+    const h = img.naturalHeight || 480;
+    if (overlay.width !== w) overlay.width = w;
+    if (overlay.height !== h) overlay.height = h;
     overlay.style.width = img.clientWidth + 'px';
     overlay.style.height = img.clientHeight + 'px';
   }
-  img.addEventListener('load', syncOverlaySize);
+  img.addEventListener('load', () => { syncOverlaySize(); drawOverlay(); });
 
   // Two-click ROI: 1st click = corner A, 2nd click = corner B (rectangle
   // defined). 3rd click starts a new selection.