The markdown editor's YAML front-matter pane was a bare textarea, so authors
had no way to discover the keys the converter honours — notably `doctype:`
(report|letter|specification) and `numbering:`, which have no other source.
Add a single server-side source of truth, convert.RecognizedFrontMatter() +
convert.FrontMatterPlaceholder(), and expose it as JSON at GET /.api/frontmatter
(handler.ServeFrontMatterTemplate; read-only, no auth — leaks only documented
field names). The browse editor fetches it once (server mode) and sets the
front-matter textarea's placeholder to the greyed hint, so an empty pane shows
the recognized keys with one-line hints. It's placeholder-only: it inserts
nothing, vanishes on the first keystroke, and arbitrary keys remain free —
front matter is still passed through to pandoc untouched. file:// mode shows no
placeholder (conversion is server-only).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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