feat: add subtasks support for tasks

[renmiamu]

Summary

This PR adds subtasks support to tasks.

Users can now:

• add subtasks to a task
• mark subtasks as completed
• remove subtasks
• see subtask completion progress in the task UI
• toggle subtask completion directly from the task details page
• view subtasks in the task details page

Motivation

The app currently supports only flat tasks. For larger todos, users often need to break a task into smaller actionable items. This feature improves planning and progress tracking.

Changes

• added subtasks to the task data model
• added a reusable subtask editor for creating and editing tasks
• displayed subtask progress in task cards
• displayed full subtask details in the task details page
• allowed toggling subtask completion directly from task details

Testing

• ran npm run build
• ran npm run test -- --run
• manually tested creating, editing, checking, and deleting subtasks

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