feat(editor): use affine container url in preview (#12919)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Improved code block preview rendering to only display the preview when
appropriate, preventing unwanted previews.

- **Refactor**
- Simplified the HTML preview system by always using a secure
iframe-based approach and removing the WebContainer integration.
- Updated iframe permissions and content delivery for enhanced security
and compatibility.

- **Chores**
- Removed the "Enable Web Container" feature flag and all related
internal logic.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -274,14 +274,6 @@ export const AFFINE_FLAGS = {
     configurable: isCanaryBuild,
     defaultState: false,
   },
-  enable_web_container: {
-    category: 'blocksuite',
-    bsFlag: 'enable_web_container',
-    displayName: 'Enable Web Container',
-    description: 'Enable web container for code block preview',
-    defaultState: false,
-    configurable: true,
-  },
 } satisfies { [key in string]: FlagInfo };
 
 // oxlint-disable-next-line no-redeclare