Refactor: Update ChevertoUploadResult model and related files

- Updated `ChevertoUploadResult.dart` to reflect the latest API response structure for Chevereto uploads.
    - Added new fields and made existing fields nullable where appropriate.
    - Introduced nested classes like `ChevertoFileObject`, `ChevertoFileResource`, `ChevertoImageDetails`, and `ChevertoFrame` for better organization.
    - Ensured `explicitToJson: true` for proper serialization of nested objects.
- Regenerated `ChevertoUploadResult.g.dart` to match the model changes.
- Enabled the "Picture Bed" settings navigation item in `SettingPage.dart`.
- Updated `ignore_for_file` comments in generated Retrofit client files to include `unused_element_parameter`.
- Removed an unused import (`flutter_cache_manager_dio`) from `DiscuzHtmlWidget.dart`.
- Removed an unused import (`cached_network_image`) from `DiscuzHtmlWidget.dart`.

Author: kidozh