Build a Markdown editor as a single-page web app in this directory: index.html, style.css, editor.js (vanilla JS, no deps). Textarea on the left, live-rendered HTML preview on the right, a toolbar (bold, italic, heading, list, link, code, quote), synced scrolling, a live word/character count, and localStorage autosave that restores on reload. Support headings, bold/italic, lists, links, inline code, code blocks, blockquotes. Polished dark theme.