- Add customIntervalDays field to Kotlin models (TaskResponse, TaskCreateRequest, TaskUpdateRequest)
- Update Android AddTaskDialog to show interval field only for "Custom" frequency
- Update Android EditTaskScreen for custom frequency support
- Update iOS TaskFormView for custom frequency support
- Fix preview data in TaskCard and TasksSection to include new field
- Add customIntervalDays to OnboardingFirstTaskView
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>