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/post/admin/routes.py

@@ -200,6 +200,63 @@ def register():
       sx_src = await render_post_data_oob(tctx)
       return sx_response(sx_src)
 
+  @bp.get("/preview/")
+  @require_admin
+  async def preview(slug: str):
+    from models.ghost_content import Post
+    from sqlalchemy import select as sa_select
+
+    from shared.sx.page import get_template_context
+    from sx.sx_components import render_post_preview_page, render_post_preview_oob
+
+    post_id = g.post_data["post"]["id"]
+    post = (await g.s.execute(
+        sa_select(Post).where(Post.id == post_id)
+    )).scalar_one_or_none()
+
+    # Build the 4 preview views
+    preview_ctx = {}
+
+    # 1. Prettified sx source
+    sx_content = getattr(post, "sx_content", None) or ""
+    if sx_content:
+        from shared.sx.prettify import sx_to_pretty_sx
+        preview_ctx["sx_pretty"] = sx_to_pretty_sx(sx_content)
+
+    # 2. Prettified lexical JSON
+    lexical_raw = getattr(post, "lexical", None) or ""
+    if lexical_raw:
+        from shared.sx.prettify import json_to_pretty_sx
+        preview_ctx["json_pretty"] = json_to_pretty_sx(lexical_raw)
+
+    # 3. SX rendered preview
+    if sx_content:
+        from shared.sx.parser import parse as sx_parse
+        from shared.sx.html import render as sx_html_render
+        from shared.sx.jinja_bridge import _COMPONENT_ENV
+        try:
+            parsed = sx_parse(sx_content)
+            preview_ctx["sx_rendered"] = sx_html_render(parsed, dict(_COMPONENT_ENV))
+        except Exception:
+            preview_ctx["sx_rendered"] = "<em>Error rendering sx</em>"
+
+    # 4. Lexical rendered preview
+    if lexical_raw:
+        from bp.blog.ghost.lexical_renderer import render_lexical
+        try:
+            preview_ctx["lex_rendered"] = render_lexical(lexical_raw)
+        except Exception:
+            preview_ctx["lex_rendered"] = "<em>Error rendering lexical</em>"
+
+    tctx = await get_template_context()
+    tctx.update(preview_ctx)
+    if not is_htmx_request():
+        html = await render_post_preview_page(tctx)
+        return await make_response(html)
+    else:
+        sx_src = await render_post_preview_oob(tctx)
+        return sx_response(sx_src)
+
   @bp.get("/entries/calendar/<int:calendar_id>/")
   @require_admin
   async def calendar_view(slug: str, calendar_id: int):