Fix page registry: escape braces for str.format()

pages_sx contains SX dict literals with {} (empty closures) which
Python's str.format() interprets as positional placeholders, causing
a KeyError that was silently caught. Escape braces before formatting.

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

shared/sx/helpers.py

@@ -771,7 +771,7 @@ def sx_page(ctx: dict, page_sx: str, *,
         component_defs=component_defs,
         styles_hash=styles_hash,
         styles_json=styles_json,
-        pages_sx=pages_sx,
+        pages_sx=pages_sx.replace("{", "{{").replace("}", "}}"),
         page_sx=page_sx,
         sx_css=sx_css,
         sx_css_classes=sx_css_classes,