Added Header Card V2 feature flags

[ronaldlangeveld]

refs https://github.com/TryGhost/Product/issues/3508

• added a feature flag for Header Cards that's being upgraded.

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🤖 Generated by Copilot at 10ac1e4

This pull request adds a new feature flag headerUpgrade to the server and the admin settings, which allows the user to opt-in to a new header design. The feature flag is only available if the labs toggle is enabled.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (ae8be7c) 66.76% compared to head (10ac1e4) 66.75%.

❗ Current head 10ac1e4 differs from pull request most recent head 18b898d. Consider uploading reports for the commit 18b898d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17263      +/-   ##
==========================================
- Coverage   66.76%   66.75%   -0.02%     
==========================================
  Files        1458     1458              
  Lines       81943    81944       +1     
  Branches    11021    11018       -3     
==========================================
- Hits        54709    54698      -11     
- Misses      26112    26125      +13     
+ Partials     1122     1121       -1     

Flag	Coverage Δ
admin-tests	36.88% <ø> (+<0.01%)	⬆️
e2e-tests	81.32% <100.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
ghost/admin/app/services/feature.js	81.35% <ø> (ø)
ghost/core/core/shared/labs.js	73.68% <100.00%> (+0.19%)	⬆️

... and 2 files with indirect coverage changes

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