Desktop workspace — folder mode + file tree sidebar

The desktop app supports a folder workspace in addition to the single-file flow. Open a folder once and every .md / .mdx / .markdown file inside is browsable in the sidebar; the choice is persisted across launches so the workspace reopens automatically next time.

Opening a folder

• Menu: File → Open Folder…
• Shortcut: Cmd/Ctrl+Shift+O
• Toolbar: 📁 Folder

Single-file open (Cmd/Ctrl+O) still works and is independent — it doesn't change the active folder.

What the sidebar shows

• A recursive tree of markdown files. Folders that contain no markdown (anywhere in their subtree) are hidden so you don't drown in node_modules clones.
• Hidden folders (anything starting with .) are skipped, with one exception: .github is included for repo workspaces.
• Standard build / dependency directories are pruned: node_modules, .git, .next, dist, out, .cache.
• Files are sorted alphabetically; folders sort before files at every level.
• Click a folder row to expand / collapse; click a file row to load it into the main view.
• The ↻ Refresh button in the sidebar header re-reads the folder tree (use it after creating/renaming files outside the app).

Persistence

The last opened folder is written to state.json in the OS user-data dir:

Platform	Path
macOS	~/Library/Application Support/Mark It Down/state.json
Windows	%APPDATA%/Mark It Down/state.json
Linux	~/.config/Mark It Down/state.json

On launch, the renderer calls mid:read-app-state and re-lists the folder tree. If the folder no longer exists, the workspace silently falls back to single-file mode.

IPC surface (preload)

Channel	Signature	Notes
mid:open-folder-dialog	() → { folderPath, tree } | null	Shows OS folder picker, persists lastFolder, returns initial tree.
mid:list-folder-md	(folderPath) → TreeEntry[]	Re-list a folder without showing a dialog (used by Refresh and on relaunch).
mid:read-app-state	() → AppState	Returns { lastFolder? }.

TreeEntry shape: { name, path, kind: 'file' | 'dir', children?: TreeEntry[] }.

Verifying

1. npm run dev:electron, then File → Open Folder… → pick this repo.
2. Sidebar lists docs/, README.md, CHANGELOG.md, etc. Click docs/ui-tokens.md — preview loads.
3. Quit and relaunch. The sidebar reopens with the same folder selected.