Bug: article_edit GET was passing raw .md file content (including YAML
frontmatter) to the body textarea. Articles synced from disk via
_sync_static_articles() had their frontmatter bled into the editor,
making it look like content was missing or garbled.
Fix: strip frontmatter (using existing _FRONTMATTER_RE) before setting
body, consistent with how _rebuild_article() already does it.
Also switch to _ARTICLES_DIR (absolute) instead of relative path.
New: split editor layout — compact metadata strip at top, dark
monospace textarea on the left, live rendered preview on the right
(HTMX, 500ms debounce). Initial preview server-rendered on page load.
New POST /admin/articles/preview endpoint returns the preview partial.
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