Add markdown task descriptions

[TN0721]

Closes #3

Adds optional multi-line task descriptions with sanitized basic Markdown rendering.

Changes:

• Adds an optional task description textarea
• Persists task descriptions in localStorage
• Renders basic Markdown for bold, italic, inline code, links, and paragraphs
• Builds Markdown output with DOM nodes so raw HTML is rendered as text
• Filters unsafe links and opens safe links in a new tab
• Adds ESLint 9 flat config so the existing lint script runs

Verification:

• npm run lint
• node --check src/app.js
• node --check src/tasks.js
• node --check src/markdown.js
• npx prettier --check index.html styles.css src/app.js src/tasks.js src/markdown.js README.md eslint.config.cjs
• git diff --check

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