Replace env free-variable threading with IO-primitive auto-fetch macros

Layout components now self-resolve context (cart-mini, auth-menu, nav-tree,
rights, URLs) via new IO primitives (root-header-ctx, select-colours,
account-nav-ctx, app-rights) and defmacro wrappers (~root-header-auto,
~auth-header-row-auto, ~root-mobile-auto). This eliminates _ctx_to_env(),
HELPER_CSS_CLASSES, and verbose :key threading across all 10 services.

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

blog/sxc/pages/helpers.py

@@ -140,7 +140,8 @@ async def _h_editor_page_content(**kw):
 
 async def _h_post_admin_content(slug=None, **kw):
     await _ensure_post_data(slug)
-    return '(div :class "pb-8")'
+    from shared.sx.helpers import render_to_sx
+    return await render_to_sx("blog-admin-placeholder")
 
 
 async def _h_post_data_content(slug=None, **kw):
@@ -264,7 +265,7 @@ async def _h_post_preview_content(slug=None, **kw):
     from quart import g
     from shared.services.registry import services
     from shared.sx.helpers import render_to_sx
-    from shared.sx.parser import SxExpr, serialize as sx_serialize
+    from shared.sx.parser import SxExpr
 
     preview = await services.blog_page.preview_data(g.s)
 
@@ -276,16 +277,16 @@ async def _h_post_preview_content(slug=None, **kw):
         sections.append(await render_to_sx("blog-preview-section",
             title="Lexical JSON", content=SxExpr(preview["json_pretty"])))
     if preview.get("sx_rendered"):
-        rendered_sx = f'(div :class "blog-content prose max-w-none" (raw! {sx_serialize(preview["sx_rendered"])}))'
+        rendered_sx = await render_to_sx("blog-preview-rendered", html=preview["sx_rendered"])
         sections.append(await render_to_sx("blog-preview-section",
             title="SX Rendered", content=SxExpr(rendered_sx)))
     if preview.get("lex_rendered"):
-        rendered_sx = f'(div :class "blog-content prose max-w-none" (raw! {sx_serialize(preview["lex_rendered"])}))'
+        rendered_sx = await render_to_sx("blog-preview-rendered", html=preview["lex_rendered"])
         sections.append(await render_to_sx("blog-preview-section",
             title="Lexical Rendered", content=SxExpr(rendered_sx)))
 
     if not sections:
-        return '(div :class "p-8 text-stone-500" "No content to preview.")'
+        return await render_to_sx("blog-preview-empty")
 
     inner = " ".join(sections)
     return await render_to_sx("blog-preview-panel", sections=SxExpr(f"(<> {inner})"))