Implement Category API methods (GetAllCategories, GetCategory)

- 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

categories.go

@@ -0,0 +1,35 @@
+package kanboard
+
+import (
+	"context"
+	"fmt"
+)
+
+// GetAllCategories returns all categories for a project.
+func (c *Client) GetAllCategories(ctx context.Context, projectID int) ([]Category, error) {
+	params := map[string]int{"project_id": projectID}
+
+	var result []Category
+	if err := c.call(ctx, "getAllCategories", params, &result); err != nil {
+		return nil, fmt.Errorf("getAllCategories: %w", err)
+	}
+
+	return result, nil
+}
+
+// GetCategory returns a category by its ID.
+// Returns ErrCategoryNotFound if the category does not exist.
+func (c *Client) GetCategory(ctx context.Context, categoryID int) (*Category, error) {
+	params := map[string]int{"category_id": categoryID}
+
+	var result *Category
+	if err := c.call(ctx, "getCategory", params, &result); err != nil {
+		return nil, fmt.Errorf("getCategory: %w", err)
+	}
+
+	if result == nil {
+		return nil, fmt.Errorf("%w: category %d", ErrCategoryNotFound, categoryID)
+	}
+
+	return result, nil
+}