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/blocks/surface/src/renderer/dom-renderer.ts

@@ -298,7 +298,10 @@ export class DomRenderer {
       viewportBounds,
       zoom
     );
-    Object.assign(domElement.style, geometricStyles);
+    const zIndexStyle = {
+      'z-index': this.layerManager.getZIndex(elementModel),
+    };
+    Object.assign(domElement.style, geometricStyles, zIndexStyle);
     Object.assign(domElement.style, PLACEHOLDER_RESET_STYLES);
 
     // Clear classes specific to shapes, if applicable
@@ -335,7 +338,10 @@ export class DomRenderer {
       zoom
     );
     const opacityStyle = getOpacity(elementModel);
-    Object.assign(domElement.style, geometricStyles, opacityStyle);
+    const zIndexStyle = {
+      'z-index': this.layerManager.getZIndex(elementModel),
+    };
+    Object.assign(domElement.style, geometricStyles, opacityStyle, zIndexStyle);
 
     this._renderElement(elementModel, domElement);
   }