Fix oscilloscope CSS: use is:global for child component styles

Astro scopes <style> block imports to the authoring component.
Since OscilloscopeDisplay.astro is a child of index.astro, the
scoped selectors didn't match. Move the CSS import into the
component itself with is:global.

frontend/src/components/OscilloscopeDisplay.astro

@@ -599,3 +599,7 @@
   // Re-init on Astro page transitions
   document.addEventListener('astro:page-load', initAll);
 </script>
+
+<style is:global>
+  @import '../styles/oscilloscope.css';
+</style>

frontend/src/pages/index.astro

@@ -24,7 +24,6 @@ try {
 >
   <style>
     @import '../styles/homepage.css';
-    @import '../styles/oscilloscope.css';
   </style>
 
   <!-- Hero: Split layout — text left, animated scope visual right -->