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>

blog/bp/blog/ghost/lexical_to_sx.py

@@ -14,6 +14,8 @@ from __future__ import annotations
 import json
 from typing import Callable
 
+import mistune
+
 
 # ---------------------------------------------------------------------------
 # Registry
@@ -435,4 +437,5 @@ def _paywall(_node: dict) -> str:
 @_converter("markdown")
 def _markdown(node: dict) -> str:
     md_text = node.get("markdown", "")
-    return f'(~kg-html :html "{_esc(md_text)}")'
+    rendered = mistune.html(md_text)
+    return f'(~kg-html :html "{_esc(rendered)}")'