api: response error in json instead of html for better rendering flexibility

internal/error/base.go

@@ -1,6 +1,7 @@
 package err
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 )
@@ -46,3 +47,18 @@ func (err baseError) Withf(format string, args ...any) Error {
 func (err *baseError) Error() string {
 	return err.Err.Error()
 }
+
+// MarshalJSON implements the json.Marshaler interface.
+func (err *baseError) MarshalJSON() ([]byte, error) {
+	//nolint:errorlint
+	switch err := err.Err.(type) {
+	case Error, *withSubject:
+		return json.Marshal(err)
+	case json.Marshaler:
+		return err.MarshalJSON()
+	case interface{ MarshalText() ([]byte, error) }:
+		return err.MarshalText()
+	default:
+		return json.Marshal(err.Error())
+	}
+}