feat: render sanitized markdown descriptions

[IsaacOdeimor]

Summary

• Adds optional multi-line task descriptions in the task form and data model.
• Renders description markdown for bold, italic, inline code, links, and line breaks.
• Sanitizes rendered output by escaping HTML and allowing only http, https, and mailto links.
• Opens rendered links in a new tab with rel="noopener noreferrer".
• Persists descriptions through the authenticated task API.

Verification

• node --test => 5 passed
• npm run lint => passed
• node --check src/markdown.js => passed
• node --check src/app.js => passed

Note: npm test timed out in this Windows shell, but the script body is node --test, which passed directly.

Closes #3.

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