Mobile app throws `cannot read property 'marginLeft' of undefined` on every book and on the settings screen

Suddenly every book fails to open with Error: cannot read property 'marginLeft' of undefined, and the settings page throws the same error, so I cannot even turn on debug logging to investigate further. It looks like it comes from the reader preferences layer rather than from my library.

Resolution: The reader preferences object can end up with preferences.layout undefined — the code reads preferences.layout.marginLeft without a nullish guard, so every screen that touches reader preferences (book view and the settings page itself) throws. Because settings are unreachable, you cannot repair it from inside the app. The reliable fix is to uninstall and reinstall the mobile app, which resets the local preferences store; your books re-download from the server and the only real loss is custom themes, which you have to recreate. Inspecting the local database afterwards showed no layout preference and no marginLeft value at all, so the bad state is an absent record rather than a corrupt value — nothing on the server needs changing.