When a user explicitly edited a task's due date, the backend was only
updating NextDueDate if the task had no completions. For recurring tasks
with completions, this caused the UI to show stale NextDueDate values
since effectiveDueDate prioritizes NextDueDate over DueDate.
Now always updates NextDueDate when user explicitly edits due date.
Completion logic will still recalculate NextDueDate when task is completed.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>