fix: resolve library dropdown empty in Docker environment
- Fix PosterManagerView library dropdown not displaying libraries when running in Docker
- Replace direct fetch() calls with configured API helper for proper base URL handling
- Ensures consistent behavior between Docker and local development environments
- Aligns with existing API patterns used by other working components (LibraryForm)
This resolves the Docker-specific issue where the library dropdown appeared empty due to improper API base URL resolution in containerized environments.