Remove wrong brace escaping from pages_sx

str.format() doesn't re-process braces inside substituted values,
so the escaping was producing literal doubled braces {{:name...}}
in the output, which the SX parser couldn't parse.

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

shared/sx/helpers.py

@@ -764,7 +764,7 @@ def sx_page(ctx: dict, page_sx: str, *,
         component_defs=component_defs,
         styles_hash=styles_hash,
         styles_json=styles_json,
-        pages_sx=pages_sx.replace("{", "{{").replace("}", "}}"),
+        pages_sx=pages_sx,
         page_sx=page_sx,
         sx_css=sx_css,
         sx_css_classes=sx_css_classes,