Add markdown task descriptions

[VexCode24]

Fixes #3.

Summary

• add an optional multi-line task description field
• render basic markdown for bold, italic, code, links, and paragraphs
• build markdown output with DOM nodes and safe URL filtering so raw HTML is shown as text and unsafe links do not get executable hrefs

Verification

• git diff --check
• node --check src/app.js src/tasks.js src/markdown.js
• browser check at http://127.0.0.1:8124/ confirmed markdown renders, safe links open in a new tab with noopener/noreferrer, and javascript: links are not executable

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @VexCode24 who has not applied for this bounty (#3). Please verify if you want to consider this contribution.

👋 Hey @VexCode24: This issue has a bounty on CodeBounty! To be eligible for the reward, please sign up on CodeBounty and apply for this bounty. If you apply, your PR will be automatically associated with your application.

[VexCode24]

Adding the exact bounty reference syntax requested on the issue: fixes #3