Can’t resolve ‘@bryntum/calendar’ when building a Modern UI customization screen (2025 R2)

I’m building a custom Modern UI screen that uses the Bryntum Calendar component uses. The screen is a view-only month calendar.
Error when publishing the customization
Module not found: Error: Can't resolve '@bryntum/calendar' in …
TS2307: Cannot find module '@bryntum/calendar' or its corresponding type declarations.
package-lock.json shows that @bryntum/calendar is linked locally, not downloaded from npm:
"node_modules/@bryntum/calendar": {
  "resolved": "node_modules/ext-packages/bryntum-calendar",
  "link": true
},
"client-controls": {
  "version": "file:..\\dist\\packages\\client-controls.tgz",
  "requires": {
    "@bryntum/calendar": "file:../ext-packages/bryntum-calendar"
  }
}