editor preview: apply github-markdown-css + highlight.js styling

[mitschabaude]

Applies GitHub-style markdown rendering to both the share viewer and the editor preview.

Share viewer

• Use github-markdown-css for GitHub-matching content styles
• Add syntax highlighting via highlight.js (GitHub theme)
• Full-width white layout, drop gray backdrop card
• Match GitHub content width (861px)
• Fix: don't parse currency amounts as inline math ($ followed by digit/comma)

Editor preview

• Import github-markdown-css/github-markdown-light.css and highlight.js/styles/github.css in src/main.tsx
• Add markdown-body class to the preview div in Editor.tsx
• Add a targeted override in styles.css to keep font/color inheriting from the card layout
• Remove the now-redundant custom heading, paragraph, pre, code, and blockquote rules

[vercel]

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Project	Deployment	Actions	Updated (UTC)
vibenote	Ready	Preview, Comment	Mar 3, 2026 4:54pm