Add IntOrFalse type to handle polymorphic API responses

Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.

types.go

@@ -2,6 +2,7 @@ package kanboard
 
 import (
 	"encoding/json"
+	"fmt"
 	"strconv"
 )
 
@@ -105,6 +106,33 @@ func (i *StringInt64) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+// IntOrFalse is an int that can be unmarshaled from a JSON int or false.
+// Kanboard API returns false on failure, int (ID) on success for create operations.
+type IntOrFalse int
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (i *IntOrFalse) UnmarshalJSON(data []byte) error {
+	// Try as int first (success case)
+	var n int
+	if err := json.Unmarshal(data, &n); err == nil {
+		*i = IntOrFalse(n)
+		return nil
+	}
+
+	// Try as bool (failure case: false)
+	var b bool
+	if err := json.Unmarshal(data, &b); err == nil {
+		if b {
+			*i = 1 // true shouldn't happen, but handle it
+		} else {
+			*i = 0
+		}
+		return nil
+	}
+
+	return fmt.Errorf("cannot unmarshal %s into IntOrFalse", data)
+}
+
 // Project represents a Kanboard project (board).
 type Project struct {
 	ID                  StringInt  `json:"id"`