fix: add seo / meta tags by front matter

[tylerdave]

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[claude]

LGTM — all three previously flagged bugs are resolved:

1. og:url construction — the template now correctly handles index slugs (stripping index and using trailing slash) and clean_urls=False (appending .html), matching the logic in get_url(). The base_url validator already strips trailing slashes, so no double-slash risk.

2. keywords as YAML list — {% if _kw is string %}...{% elif _kw %}...{% endif %} correctly handles both string and list values.

3. Folder index pages — the {% elif folder is defined %} branch now populates _title, _slug, and _fm from FolderIndex, which has a .frontmatter dict field confirmed in nav/folder_index.py:34.

No new issues found.