fix: render selection rectangle in DOM layer to appear above DOM widgets (#7474)

## Summary
Selection box was being drawn on canvas which appeared below DOM widgets
like images and textareas.

Now rendered via SelectionRectangle.vue with high z-index to ensure
visibility during drag selection.

## Screenshots (if applicable)
before
<img width="1268" height="1258" alt="image"
src="https://github.com/user-attachments/assets/7cb1271c-9ce6-4fac-83a9-ac783a309d97"
/>

after
<img width="1509" height="1129" alt="image"
src="https://github.com/user-attachments/assets/55dd698f-1213-4e60-ae46-9ed292ecd70c"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7474-fix-render-selection-rectangle-in-DOM-layer-to-appear-above-DOM-widgets-2c96d73d36508142bc2ac0d0943043c5)
by [Unito](https://www.unito.io)

src/components/graph/SelectionRectangle.vue

@@ -1,7 +1,7 @@
 <template>
   <div
     v-if="isVisible"
-    class="pointer-events-none absolute border border-blue-400 bg-blue-500/20"
+    class="pointer-events-none absolute z-9999 border border-blue-400 bg-blue-500/20"
     :style="rectangleStyle"
   />
 </template>