Preview controls float above all canvas elements, obscuring other nodes

## Problem Description

Image preview controls (and potentially other preview controls) have a z-index that causes them to float above all other elements on the canvas. This makes it possible for entire nodes to be lost/hidden beneath the layers of a preview node, significantly impacting usability.

## Visual Evidence

The issue is demonstrated in the screenshots below where nodes are completely obscured by preview layers:


## Additional Context

- This issue was identified by @without-ordinary in #7667
- The issue persists even after PR #7627 is applied (tested on commit 17be3b97)
- Screenshot after PR #7627: https://github.com/user-attachments/assets/8ddbacba-3c17-4a60-a5a1-764000c8181a

## References

- Original discussion: #7667
- Reported by: @without-ordinary
- Requested by: @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7687-Preview-controls-float-above-all-canvas-elements-obscuring-other-nodes-2cf6d73d365081238df2f7407478b2ec) by [Unito](https://www.unito.io)