Add sanitized markdown task descriptions

[Thanhdn1984]

Fixes #3

Summary

• add optional multi-line task descriptions
• render basic markdown: bold, italic, links, inline code, line breaks
• sanitize generated markup and force links to open in a new tab with noopener noreferrer

Validation

• node --check src/app.js
• node --check src/markdown.js
• node --check src/tasks.js
• python3 -m http.server 8137 + curl http://127.0.0.1:8137/

Note: npm run lint currently cannot run because the repo uses ESLint v9 but has no eslint.config.* file.

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