feat: add category write operations (create, update, remove, get-by-name)

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

categories.go

@@ -33,3 +33,80 @@ func (c *Client) GetCategory(ctx context.Context, categoryID int) (*Category, er
 
 	return result, nil
 }
+
+// CreateCategory creates a new category and returns its ID.
+func (c *Client) CreateCategory(ctx context.Context, projectID int, name string, colorID string) (int, error) {
+	params := map[string]interface{}{
+		"project_id": projectID,
+		"name":       name,
+	}
+	if colorID != "" {
+		params["color_id"] = colorID
+	}
+
+	var result IntOrFalse
+	if err := c.call(ctx, "createCategory", params, &result); err != nil {
+		return 0, fmt.Errorf("createCategory: %w", err)
+	}
+
+	if int(result) == 0 {
+		return 0, fmt.Errorf("createCategory: failed to create category %q", name)
+	}
+
+	return int(result), nil
+}
+
+// UpdateCategory updates a category's name and optionally its color.
+func (c *Client) UpdateCategory(ctx context.Context, categoryID int, name string, colorID string) error {
+	params := map[string]interface{}{
+		"id":   categoryID,
+		"name": name,
+	}
+	if colorID != "" {
+		params["color_id"] = colorID
+	}
+
+	var result bool
+	if err := c.call(ctx, "updateCategory", params, &result); err != nil {
+		return fmt.Errorf("updateCategory: %w", err)
+	}
+
+	if !result {
+		return fmt.Errorf("updateCategory: failed to update category %d", categoryID)
+	}
+
+	return nil
+}
+
+// RemoveCategory deletes a category.
+func (c *Client) RemoveCategory(ctx context.Context, categoryID int) error {
+	params := map[string]int{"category_id": categoryID}
+
+	var result bool
+	if err := c.call(ctx, "removeCategory", params, &result); err != nil {
+		return fmt.Errorf("removeCategory: %w", err)
+	}
+
+	if !result {
+		return fmt.Errorf("removeCategory: failed to remove category %d", categoryID)
+	}
+
+	return nil
+}
+
+// GetCategoryByName returns a category by name within a project.
+// Returns ErrCategoryNotFound if no category matches.
+func (c *Client) GetCategoryByName(ctx context.Context, projectID int, name string) (*Category, error) {
+	categories, err := c.GetAllCategories(ctx, projectID)
+	if err != nil {
+		return nil, err
+	}
+
+	for i := range categories {
+		if categories[i].Name == name {
+			return &categories[i], nil
+		}
+	}
+
+	return nil, fmt.Errorf("%w: category %q in project %d", ErrCategoryNotFound, name, projectID)
+}