fix(core): some editor issues (#13096)

fix AI-313, BS-3611

#### PR Dependency Tree


* **PR #13096** 👈

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

* **Refactor**
* Improved performance and resource management in code block
highlighting by using a shared highlighter instance across all code
blocks.
* Enhanced the text rendering component with additional reactive
capabilities.

* **Style**
* Updated the comment sidebar header with a new background color for
improved visual consistency.

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

packages/frontend/core/src/blocksuite/ai/components/text-renderer.ts

@@ -4,7 +4,7 @@ import { getViewManager } from '@affine/core/blocksuite/manager/view';
 import type { FeatureFlagService } from '@affine/core/modules/feature-flag';
 import { PeekViewProvider } from '@blocksuite/affine/components/peek';
 import { Container, type ServiceProvider } from '@blocksuite/affine/global/di';
-import { WithDisposable } from '@blocksuite/affine/global/lit';
+import { SignalWatcher, WithDisposable } from '@blocksuite/affine/global/lit';
 import { RefNodeSlotsProvider } from '@blocksuite/affine/inlines/reference';
 import {
   codeBlockWrapMiddleware,
@@ -106,7 +106,9 @@ export type TextRendererOptions = {
 };
 
 // todo: refactor it for more general purpose usage instead of AI only?
-export class TextRenderer extends WithDisposable(ShadowlessElement) {
+export class TextRenderer extends SignalWatcher(
+  WithDisposable(ShadowlessElement)
+) {
   static override styles = css`
     .ai-answer-text-editor.affine-page-viewport {
       background: transparent;

packages/frontend/core/src/components/comment/sidebar/style.css.ts

@@ -20,7 +20,7 @@ export const header = style({
   height: '40px',
   position: 'sticky',
   top: 0,
-  backgroundColor: cssVarV2('layer/background/overlayPanel'),
+  backgroundColor: cssVarV2('layer/background/primary'),
   zIndex: 2,
 });