fix: suppress repeating spreadsheet feature-tip popups

[interpreterwork]

Summary

• disable ONLYOFFICE feature coachmarks in offline-loader-proper.html by setting editorConfig.customization.features.featuresTips to false
• this prevents recurring "New / Cell text direction" popups from appearing each time spreadsheets are opened
• add a regression unit test that asserts featuresTips: false remains in the loader config
• include the new test in test:unit

Verification

• cd oo-editors && bun run test:unit (pass)

Related

• openinterpreter/iworkstation#521