fix(core): ai affine label alignment (#11952)

before
![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/T2klNLEk0wxLh4NRDzhk/39355c1c-a25e-4697-97aa-20a3b180d7c7.png)

after
![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/T2klNLEk0wxLh4NRDzhk/5abe900c-cd8e-43ff-bdf8-2314ccad541f.png)

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

## Summary by CodeRabbit

- **Style**
  - Updated the visual structure of the assistant avatar for a cleaner appearance.

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

packages/frontend/core/src/blocksuite/ai/chat-panel/content/assistant-avatar.ts

@@ -10,16 +10,14 @@ const AffineAvatarIcon = AiIcon({
 
 export class AssistantAvatar extends ShadowlessElement {
   static override styles = css`
-    .assistant-avatar {
+    chat-assistant-avatar {
       display: inline-flex;
       align-items: center;
       gap: 8px;
     }
   `;
   protected override render() {
-    return html`<span class="assistant-avatar"
-      >${AffineAvatarIcon} AFFiNE AI</span
-    >`;
+    return html`${AffineAvatarIcon} AFFiNE AI`;
   }
 }