Close [BS-2751](https://linear.app/affine-design/issue/BS-2751).
### What Changed?
- Do not send AI gql request when chat-panel is not open.
- When the chat-panel width is greater than 540px, adjust the padding to 24px.
- Optimize the display and hide logic of scroll to end button.
Issue:Close [BS-2150](https://linear.app/affine-design/issue/BS-2150/图片-filterprocessing-二级菜单时而不出现)
In the AI image menu (based on floating-ui):
- **Floating element**: Submenu items (e.g., "image filter/image processing")
- **Reference element**: Menu item
- **Positioning strategy**: `fixed`
- **Issue**: Specifying the `container` parameter caused the floating element (submenu) to be mounted to the reference element (menu item), leading to incorrect position calculations when floating-ui updated.
```jsx
<menu-item>
<submenu-item style={{ position: 'fixed' }}/>
</menu-item>
```
- **Fix**: Remove the `container` configuration and mount the `fixed`-positioned floating element to the `body` instead.