Show task descriptions in a dropdown

[HontoUKI]

Context

Tasks can already be created with an optional description. The frontend sends
description through createTask, and the API contract exposes it in the task
response.

Right now src/components/TaskPanel.tsx only renders the task title, so users
cannot read a saved description from the overlay.

API reference:

http://127.0.0.1:8000/docs#/default/create_task_api_tasks_post

Goal

Add a small dropdown/expand control for each task in
src/components/TaskPanel.tsx so users can open a task and read its
description.

Suggested approach

• Add local React state in TaskPanel to track which task id is expanded.
• Render a compact dropdown button for every task.
• Use aria-expanded on the dropdown button.
• When a task is expanded and task.description exists, render it below the
  task title/actions.
• If a task has no description, the expanded area can show a short empty state
  such as No description.
• Reuse the prepared CSS classes:
  • task-title
  • task-dropdown-button
  • task-description

Acceptance criteria

• Every task row has a dropdown/expand button.
• Clicking the dropdown button toggles the description for that task.
• Description text preserves line breaks and wraps inside the overlay.
• Tasks without descriptions do not break the layout.
• Existing complete/delete behavior still works.
• npm run build passes.

Files to start with

• src/components/TaskPanel.tsx
• src/styles.css
• src/types.ts