Non translatable custom field option?

### User story

It would be great to mark a custom field as non translatable which forces shopware to make it only editable on system language and otherwise only show its content.

This would be really helpful for some checkbox, switch, number fields etc. as it amkes seldom sense to translate those.

E.g. something like "Inquiry only product" which can not be bought but requires an inquiry form or other similar things and adding a OneToOne relation and additional admin component is overkill for this. However, it makes no sense to have this field have different values between e.g. English and German language.

### Acceptance criteria

- Add option per custom field to mark it as non translatable.
- Such fields can only be edited in the default language and might be displayed in other languages.

### Definition of Done

- [ ] Fulfills all acceptance criteria defined during discovery.
- [ ] Integration/E2E testing in staging is done.
- [ ] All integration/E2E/unit tests passing; all critical or high-priority bugs are resolved.
- [ ] Fulfills compliance, performance, security, and cloud-readiness needs.
- [ ] Observability—includes monitoring, alerting, and logging; incident response handbooks updated.
- [ ] Documentation—developer docs are written or updated, including information on how the feature or change is adopted and tested. If new functionality, end user documentation is written or updated.
- [ ] (conditional) All new API endpoints have a defined API schema.
- [ ] (conditional) If the change affects the Storefront, it has been tested in the latest three versions of Firefox, Chrome, Edge, IE11, and Safari, as well as the different viewports and devices (Desktop / Tablet / Mobile).
- [ ] (conditional) If the change affects the Administration, it has been tested in current versions of Firefox, Chrome, and Edge.
- [ ] (conditional) Functionality that includes new API routes are callable via the Store-API (/store-api/[version]/).
- [ ] (conditional) If a new Admin module is created, the access control layer is provided, and permissions can be managed via the Users & Permissions module.
- [ ] (conditional) If a new UI input field or interactive element is introduced, the ACL wrapper is added.
- [ ] (conditional) Performance testing done if the change has a high performance impact, such as affecting scalability, load balancing, or query optimization.

The issue names no files, tests, or entry points. Start by tracing how custom-field translations are defined and edited, then verify the system-language-only edit rule and visibility in other languages with focused tests.

Written by the indexing model from the issue text.