farhanfdjabari/notesdown — reverse-engineered prompt

Build me a simple native macOS app called NotesDown for writing Markdown files.

I want a clean split view where I type Markdown on the left and see a live preview on the right. It should work offline, open and save local Markdown files, support new windows and tabs, and handle common Markdown like headings, bold, italic, lists, quotes, code blocks, links, tables, and horizontal lines. If there’s a Mermaid flowchart code block, show a basic native preview while keeping the original text.

Please make it feel like a real Mac app with normal file dialogs, keyboard shortcuts for new, open, save, new tab, and opening multiple files, plus a toolbar toggle for light and dark theme. It should support files like md, markdown, mdown, and mkd.

Use Swift and SwiftUI for macOS 14 or later, and use Apple’s Markdown parser if needed. Keep it simple, maintainable, and include basic tests for opening, editing, saving, preview rendering, and document state.