Strip legacy CSS from SX app: no Prism, Ghost, FontAwesome extras

Add css_extras parameter to create_base_app. Legacy apps (blog, market
etc) get the default extras (basics.css, cards.css, blog-content.css,
prism.css, FontAwesome). SX app passes css_extras=[] — it uses CSSX
for styling and custom highlighting, not Prism/FA/Ghost.

Reduces <style id="sx-css"> from ~100KB+ of irrelevant CSS to ~5KB
of Tailwind resets + only the utility rules the page actually uses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/app.py

@@ -64,6 +64,7 @@ def create_app() -> "Quart":
         "sx",
         context_fn=sx_standalone_context if SX_STANDALONE else sx_docs_context,
         domain_services_fn=register_domain_services,
+        css_extras=[],  # No legacy CSS — SX uses CSSX + custom highlighting
         **extra_kw,
     )