Implement comprehensive error types and handling

Add complete error handling system for Kanboard API client:

Sentinel errors:
- Network: ErrConnectionFailed, ErrTimeout
- Auth: ErrUnauthorized, ErrForbidden
- Resources: ErrNotFound, ErrProjectNotFound, ErrTaskNotFound,
  ErrColumnNotFound, ErrCommentNotFound
- Logic: ErrAlreadyInLastColumn, ErrAlreadyInFirstColumn,
  ErrTaskClosed, ErrTaskOpen
- Validation: ErrEmptyTitle, ErrInvalidProjectID

Helper functions:
- IsNotFound() - checks all not-found error variants
- IsUnauthorized() - checks auth errors
- IsAPIError() - checks for API errors via errors.As

All errors support errors.Is/errors.As for proper error
wrapping and context preservation.

Closes: kanboard-api-s7k

errors.go

@@ -5,10 +5,17 @@ import (
 	"fmt"
 )
 
+// Network errors
 var (
 	// ErrConnectionFailed indicates a connection to the Kanboard server failed.
 	ErrConnectionFailed = errors.New("connection to Kanboard server failed")
 
+	// ErrTimeout indicates a request timed out.
+	ErrTimeout = errors.New("request timed out")
+)
+
+// Authentication errors
+var (
 	// ErrUnauthorized indicates authentication failed.
 	ErrUnauthorized = errors.New("authentication failed: invalid credentials")
 
@@ -16,6 +23,48 @@ var (
 	ErrForbidden = errors.New("access forbidden: insufficient permissions")
 )
 
+// Resource errors
+var (
+	// ErrNotFound indicates a resource was not found.
+	ErrNotFound = errors.New("resource not found")
+
+	// ErrProjectNotFound indicates the specified project was not found.
+	ErrProjectNotFound = errors.New("project not found")
+
+	// ErrTaskNotFound indicates the specified task was not found.
+	ErrTaskNotFound = errors.New("task not found")
+
+	// ErrColumnNotFound indicates the specified column was not found.
+	ErrColumnNotFound = errors.New("column not found")
+
+	// ErrCommentNotFound indicates the specified comment was not found.
+	ErrCommentNotFound = errors.New("comment not found")
+)
+
+// Logic errors
+var (
+	// ErrAlreadyInLastColumn indicates a task is already in the last column.
+	ErrAlreadyInLastColumn = errors.New("task is already in the last column")
+
+	// ErrAlreadyInFirstColumn indicates a task is already in the first column.
+	ErrAlreadyInFirstColumn = errors.New("task is already in the first column")
+
+	// ErrTaskClosed indicates a task is already closed.
+	ErrTaskClosed = errors.New("task is already closed")
+
+	// ErrTaskOpen indicates a task is already open.
+	ErrTaskOpen = errors.New("task is already open")
+)
+
+// Validation errors
+var (
+	// ErrEmptyTitle indicates a task title cannot be empty.
+	ErrEmptyTitle = errors.New("task title cannot be empty")
+
+	// ErrInvalidProjectID indicates an invalid project ID was provided.
+	ErrInvalidProjectID = errors.New("invalid project ID")
+)
+
 // APIError represents an error returned by the Kanboard API.
 type APIError struct {
 	Code    int
@@ -26,3 +75,23 @@ type APIError struct {
 func (e *APIError) Error() string {
 	return fmt.Sprintf("Kanboard API error (code %d): %s", e.Code, e.Message)
 }
+
+// IsNotFound returns true if the error indicates a resource was not found.
+func IsNotFound(err error) bool {
+	return errors.Is(err, ErrNotFound) ||
+		errors.Is(err, ErrProjectNotFound) ||
+		errors.Is(err, ErrTaskNotFound) ||
+		errors.Is(err, ErrColumnNotFound) ||
+		errors.Is(err, ErrCommentNotFound)
+}
+
+// IsUnauthorized returns true if the error indicates an authentication failure.
+func IsUnauthorized(err error) bool {
+	return errors.Is(err, ErrUnauthorized)
+}
+
+// IsAPIError returns true if the error is an APIError from the Kanboard API.
+func IsAPIError(err error) bool {
+	var apiErr *APIError
+	return errors.As(err, &apiErr)
+}