✨ feat: add image upload toggle with auto-disable after sending

Add a toggle switch to enable/disable image uploads in the playground,
with automatic disabling after message sending to prevent accidental
image inclusion in subsequent messages.

Changes:
- Add `imageEnabled` field to default configuration with false as default
- Enhance ImageUrlInput component with enable/disable toggle switch
- Update UI to show disabled state with opacity and color changes
- Modify message sending logic to only include images when enabled
- Implement auto-disable functionality after message is sent
- Update SettingsPanel to pass through new imageEnabled props
- Maintain backward compatibility with existing configurations

User Experience:
- Images are disabled by default for privacy and intentional usage
- Users must explicitly enable image uploads before adding URLs
- After sending a message with images, the feature auto-disables
- Clear visual feedback shows current enabled/disabled state
- Manual control allows users to re-enable when needed

This improves user control over multimodal conversations and prevents
unintentional image sharing in follow-up messages.

web/src/components/playground/SettingsPanel.js

@@ -125,7 +125,9 @@ const SettingsPanel = ({
         {/* 图片URL输入 */}
         <ImageUrlInput
           imageUrls={inputs.imageUrls}
+          imageEnabled={inputs.imageEnabled}
           onImageUrlsChange={(urls) => onInputChange('imageUrls', urls)}
+          onImageEnabledChange={(enabled) => onInputChange('imageEnabled', enabled)}
         />
 
         {/* 参数控制组件 */}