Release new admin shell and sidebar

[jonatansberg]

closes https://linear.app/ghost/issue/BER-3230/release-new-admin-shell

Removed the feature flagging and conditionals so that we always serve the new admin shell. This includes the new sidebar UI and a significantly improved local developer experience via yarn dev:forward (soon to be renamed yarn dev.)

[coderabbitai]

Walkthrough

This pull request removes the adminForward (New Admin Experience) feature flag across the codebase: deletes its settings UI entry; stops generating/serving a separate index-forward.html (build now uses index.html and overwrites it); removes automatic environment injection in e2e setup; deletes the adminForward property and reconciliation logic from the admin feature service (including cookie utilities and fetch-time invocation); simplifies the admin controller to always serve index.html; and removes adminForward from the private labs feature list.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Release new admin shell and sidebar' accurately summarizes the main objective of removing feature flags and conditionals to always serve the new admin shell with sidebar UI.
Description check	✅ Passed	The description clearly explains the purpose of removing feature flagging so the app always serves the new admin shell, including the sidebar UI and developer experience improvements.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.51%. Comparing base (92091de) to head (2dc087c).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25955      +/-   ##
==========================================
- Coverage   72.55%   72.51%   -0.05%     
==========================================
  Files        1546     1548       +2     
  Lines      118601   119027     +426     
  Branches    14306    14359      +53     
==========================================
+ Hits        86053    86312     +259     
- Misses      31508    31692     +184     
+ Partials     1040     1023      -17     

Flag	Coverage Δ
admin-tests	51.81% <ø> (+0.01%)	⬆️
e2e-tests	72.51% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[cmraible]

Does this mean we can remove the duplicate e2e tests for Ember in CI now? No opinion on whether that should be included here or in a follow up, just curious and excited to simplify that 😄

[jonatansberg]

Yeah, thats correct! I have a separate tasks for that since there are some additional cleanup that goes along with it.