fix(ui): restore missing resize handles for data explorer panel

steps/index.html

@@ -667,6 +667,16 @@
         <button id="plot-selected-btn"
           class="bg-blue-600 text-white px-3 py-1 rounded-lg text-sm hover:bg-blue-700">Plot Selected Column</button>
       </div>
+
+      <!-- Resizers -->
+      <div class="resizer resizer-t"></div>
+      <div class="resizer resizer-r"></div>
+      <div class="resizer resizer-b"></div>
+      <div class="resizer resizer-l"></div>
+      <div class="resizer resizer-tl"></div>
+      <div class="resizer resizer-tr"></div>
+      <div class="resizer resizer-br"></div>
+      <div class="resizer resizer-bl"></div>
     </div>
   </main>