Add markdown task descriptions

[nguyenduc071912]

Summary

Adds optional task descriptions with safe basic markdown rendering.

• Adds a multi-line description field to the task form
• Persists trimmed descriptions through local and authenticated task storage
• Renders basic markdown for bold, italic, inline code, links, and line breaks
• Escapes HTML before markdown conversion and only links safe URL protocols
• Opens rendered links in a new tab with rel="noopener noreferrer"
• Extends server tests for persisted descriptions

fixes #3

Tests

npm test
npm run lint

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