Add admin preview views + fix markdown converter

- Fix _markdown() in lexical_to_sx.py: render markdown to HTML with
  mistune.html() before storing in ~kg-html
- Add shared/sx/prettify.py: sx_to_pretty_sx and json_to_pretty_sx
  produce sx AST for syntax-highlighted DOM (uses canonical serialize())
- Add preview tab to admin header nav
- Add GET /preview/ route with 4 views: prettified sx, prettified
  lexical JSON, sx rendered HTML, lexical rendered HTML
- Add ~blog-preview-panel and ~blog-preview-section components
- Add syntax highlight CSS for sx/JSON tokens

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/helpers.py

@@ -205,6 +205,7 @@ def post_admin_header_sx(ctx: dict, slug: str, *, oob: bool = False,
         ("cart_url",   f"/{slug}/admin/payments/", "payments"),
         ("blog_url",   f"/{slug}/admin/entries/", "entries"),
         ("blog_url",   f"/{slug}/admin/data/", "data"),
+        ("blog_url",   f"/{slug}/admin/preview/", "preview"),
         ("blog_url",   f"/{slug}/admin/edit/", "edit"),
         ("blog_url",   f"/{slug}/admin/settings/", "settings"),
     ]