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/framework/std/src/gfx/layer.ts

@@ -57,7 +57,7 @@ export type CanvasLayer = BaseLayer<GfxPrimitiveElementModel> & {
   type: 'canvas';
 
   /**
-   * The z-index of canvas layer.
+   * The z-index of the first element in this canvas layer.
    *
    * A canvas layer renders all the elements in a single canvas,
    *  this property is used to render the canvas with correct z-index.
@@ -165,8 +165,7 @@ export class LayerManager extends GfxExtension {
         ];
         curLayer.zIndex = currentCSSZindex;
         layers.push(curLayer as LayerManager['layers'][number]);
-        currentCSSZindex +=
-          curLayer.type === 'block' ? curLayer.elements.length : 1;
+        currentCSSZindex += curLayer.elements.length;
       }
     };
     const addLayer = (type: 'canvas' | 'block') => {

blocksuite/framework/std/src/utils/layer.ts

@@ -12,11 +12,7 @@ import type { SurfaceBlockModel } from '../gfx/model/surface/surface-model.js';
 
 export function getLayerEndZIndex(layers: Layer[], layerIndex: number) {
   const layer = layers[layerIndex];
-  return layer
-    ? layer.type === 'block'
-      ? layer.zIndex + layer.elements.length - 1
-      : layer.zIndex
-    : 0;
+  return layer ? layer.zIndex + layer.elements.length - 1 : 0;
 }
 
 export function updateLayersZIndex(layers: Layer[], startIdx: number) {
@@ -27,7 +23,7 @@ export function updateLayersZIndex(layers: Layer[], startIdx: number) {
     const curLayer = layers[i];
 
     curLayer.zIndex = curIndex;
-    curIndex += curLayer.type === 'block' ? curLayer.elements.length : 1;
+    curIndex += curLayer.elements.length;
   }
 }