Two improvements to browse's preview-markdown plugin so it can replace
the standalone mdedit tool:
1. **YAML front-matter editing.** The FM pane above the outline used to
render a read-only <dl> of parsed keys — sparse and unusable when
the file had no envelope yet. It's now a dedicated <textarea> that's
always present. On load, parseFrontMatter() splits the `---\n…\n---`
envelope off the body: the body feeds Toast UI Editor, the envelope
feeds the textarea. On save, assembleContent() recombines them.
Dirty tracking covers both halves via a SHA-256 of the assembled
bytes. The shell mirrors mdedit's old layout (FM textarea top,
outline below) but the FM pane is now always functional, eliminating
the "empty pane over the TOC" problem.
2. **Download as DOCX / HTML / PDF.** When the file handle is HTTP-
backed (server mode) and the file is a .md, three buttons appear in
the info header next to Save. Clicking one fetches the server's
?convert=<fmt> endpoint and triggers a browser download with a
clean filename (foo.md → foo.docx). Auto-saves the buffer first if
dirty so the converted bytes reflect what's on screen.
Helper at window.zddc.source.downloadConverted (shared/zddc-source.js)
so other tools — archive, transmittal — can reuse the same flow later.
Friendly error messages map HTTP 503 / 422 / 504 to actionable toasts.
ZDDC
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