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>

shared/infrastructure/factory.py

@@ -49,6 +49,7 @@ def create_base_app(
     domain_services_fn: Callable[[], None] | None = None,
     no_oauth: bool = False,
     no_db: bool = False,
+    css_extras: Sequence[str] | None = None,
 ) -> Quart:
     """
     Create a Quart app with shared infrastructure.
@@ -139,17 +140,24 @@ def create_base_app(
     _styles = BASE_DIR / "static" / "styles"
     _fa_css = BASE_DIR / "static" / "fontawesome" / "css"
     if (_styles / "tw.css").exists() and not registry_loaded():
-        load_css_registry(
-            _styles / "tw.css",
-            extra_css=[
+        if css_extras is None:
+            # Legacy default: all shared CSS for blog/market/etc apps
+            _extra = [
                 _styles / "basics.css",
                 _styles / "cards.css",
                 _styles / "blog-content.css",
                 _styles / "prism.css",
                 _fa_css / "all.min.css",
                 _fa_css / "v4-shims.min.css",
-            ],
-            url_rewrites={"../webfonts/": "/static/fontawesome/webfonts/"},
+            ]
+            _rewrites = {"../webfonts/": "/static/fontawesome/webfonts/"}
+        else:
+            _extra = [_styles / e if "/" not in e else e for e in css_extras]
+            _rewrites = {}
+        load_css_registry(
+            _styles / "tw.css",
+            extra_css=_extra,
+            url_rewrites=_rewrites,
         )
 
     # Dev-mode: auto-reload sx templates when files change on disk

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,
     )