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.
The primary purpose of this PR appears to be:
1. Simplifying the change notification API by removing the redundant value parameter from callbacks
2. Improving the reactive system's handling of specialized types (Text and Boxed) in flat data
3. Adding better test coverage for text handling in the flat data model