refactor: standardize z-index Tailwind classes (#7509)

## Summary

Replace arbitrary z-index values (z-[n]) with standard Tailwind
utilities.

fix https://github.com/Comfy-Org/ComfyUI_frontend/issues/7499

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7509-refactor-standardize-z-index-Tailwind-classes-2ca6d73d365081339e66f0c8e6fcc20b)
by [Unito](https://www.unito.io)

src/components/graph/GraphCanvasMenu.vue

@@ -10,7 +10,7 @@
     ></div>
 
     <ButtonGroup
-      class="absolute right-0 bottom-0 z-[1200] flex-row gap-1 border-[1px] border-interface-stroke bg-comfy-menu-bg p-2"
+      class="absolute right-0 bottom-0 z-1200 flex-row gap-1 border-[1px] border-interface-stroke bg-comfy-menu-bg p-2"
       :style="{
         ...stringifiedMinimapStyles.buttonGroupStyles
       }"

src/components/graph/selectionToolbox/SubmenuPopover.vue

@@ -8,7 +8,7 @@
     unstyled
     :pt="{
       root: {
-        class: 'absolute z-[60]'
+        class: 'absolute z-60'
       },
       content: {
         class: [