Fixed Koenig editor failing to load in React admin shell

[cmraible]

ref https://linear.app/ghost/issue/BER-3111/

The Koenig editor was not loading when using yarn dev:forward because Settings.tsx was passing an empty no-op function for fetchKoenigLexical.

After implementing a proper fetchKoenigLexical function that loaded the UMD bundle from /ghost/assets/koenig-lexical/, a second issue emerged: React hooks errors ("Invalid hook call", "Cannot read properties of null reading 'useMemo'"). This happened because the UMD bundle includes its own bundled React, which conflicted with Vite's React instance in dev mode.

The fix imports @tryghost/koenig-lexical as an ESM module directly, allowing Vite to handle React deduplication. This required adding the package as a dependency to the admin app.

Changes:

• Added @tryghost/koenig-lexical dependency to apps/admin
• Created fetchKoenigLexical.ts utility that imports the ESM module
• Updated Settings.tsx to use the new utility function

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix-koenig-editor-dev-forward

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.11%. Comparing base (2ace1fa) to head (5a254af).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25675      +/-   ##
==========================================
+ Coverage   72.02%   72.11%   +0.09%     
==========================================
  Files        1533     1535       +2     
  Lines      117438   117582     +144     
  Branches    14040    14111      +71     
==========================================
+ Hits        84580    84800     +220     
+ Misses      31850    31774      -76     
  Partials     1008     1008              

Flag	Coverage Δ
admin-tests	51.80% <ø> (+0.01%)	⬆️
e2e-tests	72.11% <ø> (+0.09%)	⬆️

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[cmraible]

Tested this out and it's working locally, but I'm still getting the red "An error occurred loading editor. Please refresh and try again." error when I tried deploying this to staging. Not sure if it's an issue with the code in this PR, or if we're maybe not versioning the assets properly, so deploying a PR to a staging site isn't using the correct files.