Refactor models to replace deprecated `@ember/object/computed` macros with `@dependentKeyCompat` and native class fields powered by `@tracked`. This aligns with modern Ember patterns, ensuring future compatibility and performance improvements.

Update `site.js` to use `isReadOnly` instead of `is_readonly`, aligning with standard camelCase conventions. Removed the legacy getter for `isReadOnly`, as it is no longer needed with `@tracked` properties. This simplifies code and adheres to modern Ember best practices.

Update `composer-body.js` to replace the key name `composer.sharedDraft`with `composer.creatingSharedDraft`, improving clarity and consistencywithin the `bindProperties` list. Ensures better readability and alignswith naming conventions across the component.

Ensure `AUTO_GROUPS.everyone` is passed as an array in the header API testto comply with expected `user.groups` formatting. This change improvestest stability and consistency with the `needs.user` API usage.

Update the `sharedDraft` key to `creatingSharedDraft` in `composer.js` forimproved clarity and consistency with the naming conventions used acrossthe codebase. This change ensures better readability and alignment withexisting patterns.