feat(dashboard): add localized content language controls

[dlhck]

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 #4972 and relates to #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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[vercel]

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Project	Deployment	Actions	Updated (UTC)
vendure-storybook	Ready	Preview, Comment	Jul 23, 2026 12:12pm

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[github-actions]

Dashboard Preview: https://admin-dashboard-ofm9hpedk-vendure.vercel.app

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.8% Duplication on New Code

See analysis details on SonarQube Cloud