fix: ask ai panel not auto flip (#7284)

Close [AF-928](https://linear.app/affine-design/issue/AF-928/ask-ai-%E5%87%BA%E7%8E%B0%E5%9C%A8%E5%BA%95%E9%83%A8%E7%9A%84%E6%97%B6%E5%80%99-menu-%E5%BC%B9%E5%87%BA%E4%BD%8D%E7%BD%AE%E4%B8%8D%E5%AF%B9)

The CSS rule for portal content uses `position: absolute` which causes the portal size to become zero. As a result, the `computePosition` function returns incorrect results.

Before:
![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/MyRfgiN4RuBxJfrza3SG/5ae1d556-d818-42d4-a84c-ee5531015182.png)

After:
![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/MyRfgiN4RuBxJfrza3SG/a61c47bc-541a-499a-8f20-3b3dd03b4e1d.png)

packages/frontend/core/src/blocksuite/presets/ai/_common/components/ask-ai-button.ts

@@ -104,8 +104,8 @@ export class AskAIButton extends WithDisposable(LitElement) {
         ></ask-ai-panel>`,
         computePosition: {
           referenceElement: this,
-          placement: 'top-start',
-          middleware: [flip(), offset(-40)],
+          placement: 'bottom-start',
+          middleware: [flip(), offset({ mainAxis: 8, crossAxis: -6 })],
           autoUpdate: true,
         },
       };
@@ -160,7 +160,7 @@ export class AskAIButton extends WithDisposable(LitElement) {
       computePosition: {
         referenceElement: this._askAIButton,
         placement: 'bottom-start',
-        middleware: [flip(), offset(4)],
+        middleware: [flip(), offset({ mainAxis: 8, crossAxis: -6 })],
         autoUpdate: true,
       },
       abortController: this._abortController,

packages/frontend/core/src/blocksuite/presets/ai/_common/components/ask-ai-panel.ts

@@ -13,7 +13,7 @@ import { getRootService } from '../../utils/selection-utils';
 export class AskAIPanel extends WithDisposable(LitElement) {
   static override styles = css`
     :host {
-      position: absolute;
+      position: relative;
     }
 
     .ask-ai-panel {