Add CreateCategory, UpdateCategory, RemoveCategory, and GetCategoryByName
methods to the client, with BoardScope wrappers for project-scoped operations.
Includes comprehensive tests for all new methods.
Closes kanboard-4n3
- GetAllCategories returns all categories for a project
- GetCategory returns a single category by ID
- Returns ErrCategoryNotFound for non-existent categories
- Added ErrCategoryNotFound to IsNotFound helper
- Comprehensive test coverage
