Remove render_to_sx from public API: enforce sx_call for all service code

Replace ~250 render_to_sx calls across all services with sync sx_call,
converting many async functions to sync where no other awaits remained.
Make render_to_sx/render_to_sx_with_env private (_render_to_sx).
Add (post-header-ctx) IO primitive and shared post/post-admin defmacros.
Convert built-in post/post-admin layouts from Python to register_sx_layout
with .sx defcomps. Remove dead post_admin_mobile_nav_sx.

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

shared/browser/app/errors.py

@@ -149,7 +149,7 @@ async def _rich_error_page(errnum: str, message: str, image: str | None = None)
         # Root header (site nav bar)
         from shared.sx.helpers import (
             root_header_sx, post_header_sx,
-            header_child_sx, full_page_sx, render_to_sx,
+            header_child_sx, full_page_sx, sx_call,
         )
         hdr = await root_header_sx(ctx)
 
@@ -162,7 +162,7 @@ async def _rich_error_page(errnum: str, message: str, image: str | None = None)
                 hdr = "(<> " + hdr + " " + await header_child_sx(post_row) + ")"
 
         # Error content
-        error_content = await render_to_sx("error-content", errnum=errnum, message=message, image=image)
+        error_content = sx_call("error-content", errnum=errnum, message=message, image=image)
 
         return await full_page_sx(ctx, header_rows=hdr, content=error_content)
     except Exception: