feat: add backend part to support sections (#577)

Author: perber

internal/core/tree/errors.go

@@ -1,6 +1,9 @@
 package tree
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 var ErrPageNotFound = errors.New("page not found")
 var ErrParentNotFound = errors.New("parent not found")
@@ -12,3 +15,69 @@ var ErrPageCannotBeMovedToItself = errors.New("page cannot be moved to itself")
 var ErrInvalidSortOrder = errors.New("invalid sort order")
 var ErrFrontmatterParse = errors.New("frontmatter parse error")
 var ErrFileNotFound = errors.New("file not found")
+var ErrDrift = errors.New("drift detected")
+var ErrInvalidOperation = errors.New("invalid operation")
+var ErrConvertNotAllowed = errors.New("convert not allowed")
+
+// DriftError represents a drift error with detailed information.
+type DriftError struct {
+	NodeID string
+	Kind   NodeKind
+	Path   string
+	Reason string
+}
+
+func (e *DriftError) Error() string {
+	return "drift detected: nodeID=" + e.NodeID + ", kind=" + string(e.Kind) + ", path=" + e.Path + ", reason=" + e.Reason
+}
+
+func (e *DriftError) Unwrap() error {
+	return ErrDrift
+}
+
+// InvalidOpError represents an invalid operation error with details.
+type InvalidOpError struct {
+	Op     string
+	Reason string
+}
+
+func (e *InvalidOpError) Error() string { return fmt.Sprintf("%s: %s", e.Op, e.Reason) }
+func (e *InvalidOpError) Unwrap() error { return ErrInvalidOperation }
+
+// PageAlreadyExistsError: Konflikt bei Create/Move/Rename
+type PageAlreadyExistsError struct {
+	Path string
+}
+
+func (e *PageAlreadyExistsError) Error() string { return fmt.Sprintf("already exists: %s", e.Path) }
+func (e *PageAlreadyExistsError) Unwrap() error { return ErrPageAlreadyExists }
+
+// NotFoundError represents a not found error with details.
+type NotFoundError struct {
+	Resource string
+	ID       string
+	Path     string
+}
+
+func (e *NotFoundError) Error() string {
+	return fmt.Sprintf("%s not found: %s", e.Resource, e.ID)
+}
+
+func (e *NotFoundError) Unwrap() error {
+	return ErrPageNotFound
+}
+
+// ConvertNotAllowedError represents a convert not allowed error with details.
+type ConvertNotAllowedError struct {
+	From   NodeKind
+	To     NodeKind
+	Reason string
+}
+
+func (e *ConvertNotAllowedError) Error() string {
+	return fmt.Sprintf("cannot convert from %s to %s: %s", e.From, e.To, e.Reason)
+}
+
+func (e *ConvertNotAllowedError) Unwrap() error {
+	return ErrConvertNotAllowed
+}