fix(editor): incorrect z-index in dom renderer (#13465)

#### PR Dependency Tree


* **PR #13464**
  * **PR #13465** 👈
    * **PR #13471**
      * **PR #13472**
        * **PR #13473**

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved stacking order across canvas elements (shapes, connectors,
brush, highlighter), reducing unexpected overlap.
* Corrected z-index application for placeholders and fully rendered
elements to ensure consistent layering during edits.
* **Refactor**
* Centralized z-index handling for canvas elements to provide
predictable, uniform layering behavior across the app.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/gfx/brush/src/renderer/dom/brush.ts

@@ -65,8 +65,5 @@ export const BrushDomRendererExtension = DomElementRendererExtension(
     domElement.style.height = `${h * zoom}px`;
     domElement.style.overflow = 'visible';
     domElement.style.pointerEvents = 'none';
-
-    // Set z-index for layering
-    domElement.style.zIndex = renderer.layerManager.getZIndex(model).toString();
   }
 );

blocksuite/affine/gfx/brush/src/renderer/dom/highlighter.ts

@@ -69,8 +69,5 @@ export const HighlighterDomRendererExtension = DomElementRendererExtension(
     domElement.style.height = `${h * zoom}px`;
     domElement.style.overflow = 'visible';
     domElement.style.pointerEvents = 'none';
-
-    // Set z-index for layering
-    domElement.style.zIndex = renderer.layerManager.getZIndex(model).toString();
   }
 );