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 -->

blocksuite/affine/shared/src/services/feature-flag-service.ts

@@ -21,7 +21,6 @@ export interface BlockSuiteFlags {
   enable_table_virtual_scroll: boolean;
   enable_turbo_renderer: boolean;
   enable_dom_renderer: boolean;
-  enable_web_container: boolean;
 }
 
 export class FeatureFlagService extends StoreExtension {
@@ -47,7 +46,6 @@ export class FeatureFlagService extends StoreExtension {
     enable_table_virtual_scroll: false,
     enable_turbo_renderer: false,
     enable_dom_renderer: false,
-    enable_web_container: false,
   });
 
   setFlag(key: keyof BlockSuiteFlags, value: boolean) {