Allow clicking different tool while detail panel is open (THE-117)

The panel overlay (z-index 199) was intercepting all clicks on the tree
when the panel was open, forcing users to close the panel before selecting
a different tool. Setting pointer-events: none on the overlay lets clicks
pass through to tree nodes, where openPanel() already handles replacing
the panel content. The SVG background click handler still closes the panel
when clicking empty canvas.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

public/css/panel.css

@@ -11,6 +11,7 @@
 
 #panel-overlay.visible {
   display: block;
+  pointer-events: none;
 }
 
 /* Panel container */

public/js/arf.js

@@ -589,13 +589,10 @@ document.addEventListener("keydown", function(e) {
   if (e.key === "Escape") closePanel();
 });
 
-// Wire close button and overlay once DOM is ready
+// Wire close button once DOM is ready
 document.addEventListener("DOMContentLoaded", function() {
   var closeBtn = document.getElementById("panel-close");
   if (closeBtn) closeBtn.addEventListener("click", closePanel);
-
-  var overlay = document.getElementById("panel-overlay");
-  if (overlay) overlay.addEventListener("click", closePanel);
 });
 
 // Canvas click: close panel when clicking the SVG background (not a node)