Keep styles.css hidden in sandboxes

The Pages Router build always hid the generated /src/styles.css tab
(the old check read a `visible` flag that was never set). Restoring that
so ~76 pages don't grow a second tab.

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

Author: gaearon

src/components/MDX/Sandpack/SandpackRSCRoot.tsx

@@ -90,7 +90,7 @@ function SandpackRSCRoot(props: SandpackProps) {
 
   files['/src/styles.css'] = {
     code: [sandboxStyle, files['/src/styles.css']?.code ?? ''].join('\n\n'),
-    hidden: sourceFiles['/src/styles.css']?.hidden ?? true,
+    hidden: true,
   };
 
   return (

src/components/MDX/Sandpack/SandpackRoot.tsx

@@ -87,7 +87,7 @@ function SandpackRoot(props: SandpackProps) {
 
   files['/src/styles.css'] = {
     code: [sandboxStyle, files['/src/styles.css']?.code ?? ''].join('\n\n'),
-    hidden: sourceFiles['/src/styles.css']?.hidden ?? true,
+    hidden: true,
   };
 
   return (