Added welcomeEmailDesignCustomization feature flag

[cmraible]

Summary

• Adds a new welcomeEmailDesignCustomization private feature flag to enable toggling welcome email design customization on/off
• Adds the flag to the labs settings UI under private features
• Updates the config API snapshot to include the new flag

refs https://linear.app/ghost/issue/NY-1106

Test plan

• Unit tests pass (ghost/core/test/unit/shared/labs.test.js — 10 passing)
• Config API e2e snapshot updated and passing
• Toggle visible in Labs → Private features when developer experiments is enabled

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 65c3dff and e92e2bf.

⛔ Files ignored due to path filters (1)

• ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap is excluded by !**/*.snap

📒 Files selected for processing (2)

• apps/admin-x-settings/src/components/settings/advanced/labs/private-features.tsx
• ghost/core/core/shared/labs.js

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Walkthrough

This change introduces a new private lab feature for welcome email design customization. The feature flag welcomeEmailDesignCustomization is added to enable design customization options for welcome emails. The implementation spans two files: the private features configuration in the admin settings UI component where the feature entry with title and description is defined, and the core labs module where the feature flag is registered in the PRIVATE_FEATURES array alongside existing feature flags. This allows the feature to be toggled as a private lab experiment within the system.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Added welcomeEmailDesignCustomization feature flag' clearly and concisely describes the main change—addition of a new feature flag—matching the changeset perfectly.
Description check	✅ Passed	The description is directly related to the changeset, providing context about the new feature flag addition, its purpose, and testing status.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch chris-ny-1106-create-feature-flag-for-email-settings-customization

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