Fix markdown rendering in task descriptions

[Thanhdn1984]

Summary

• add optional multi-line task descriptions
• render a safe subset of markdown (bold, italic, code, links)
• sanitize rendering by building DOM nodes directly and allowing only http/https/mailto links
• open rendered links in a new tab with rel="noopener noreferrer"
• persist descriptions through the existing task API

Tests

• npm test
• npm run lint

Closes #3

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