[upstream #5000] feat(dashboard): add localized content language controls

[ayim]

Mirrored from vendurehq#5000 for the Overwatch review demo.

Original author: @dlhck

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Description

Adds independent, form-local language controls for all built-in translatable Dashboard detail forms and localized custom fields while preserving the global content language for surrounding API-translated content. Translation drafts are limited to active-channel languages, out-of-channel translations are preserved, and empty no-ID drafts are removed before submission. Validation covers focused Vitest suites plus the products, custom-fields, and fallback-placeholder Playwright suites. Fixes vendurehq#4972 and relates to vendurehq#4885.

Breaking changes

No breaking changes.

Screenshots

Not included.

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed

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