feat(biochem): surface 3D toggle button in molecular editor

3D rendering (toggle3D/render3D with VDW radii + sphere gradients) was
implemented but had no visible Esc-key exit and cursor was left in 'grab'
state when leaving 3D mode. Changes:
- Esc key now exits 3D mode (toggle3D) in addition to cancel/close actions
- cursor correctly resets to 'crosshair' on 3D exit and 'grab' on enter
- btn-3d toolbar button confirmed visible with mode-3d-active active state

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/biochem.html

@@ -1678,10 +1678,12 @@ function toggle3D() {
   vdwBtn.style.display = _is3D ? '' : 'none';
   if (!_is3D && _isVDW) { _isVDW = false; vdwBtn.classList.remove('mode-3d-active'); }
   if (_is3D) {
+    canvas.style.cursor = 'grab';
     centerView();
     _start3D();
   } else {
     _stop3D();
+    canvas.style.cursor = 'crosshair';
     render();
   }
 }