refactor(MermaidDiagramFrame): remove theme synchronization in fullscreen mode

Eliminate the useEffect that mirrored the document's data-theme attribute onto the fullscreen root, simplifying the component's logic. This change enhances performance by reducing unnecessary observers and event listeners.

Made-with: Cursor

Author: danbaruka

website/src/components/MermaidDiagramFrame/index.tsx

@@ -61,30 +61,6 @@ export default function MermaidDiagramFrame({ children, hint }: MermaidDiagramFr
     return () => document.removeEventListener('fullscreenchange', onFs);
   }, []);
 
-  /** Mirror site light/dark onto the fullscreen root so backgrounds and color-scheme stay correct. */
-  useEffect(() => {
-    const el = rootRef.current;
-    if (!el) return;
-
-    if (!fullscreen) {
-      el.removeAttribute('data-theme');
-      return;
-    }
-
-    const syncTheme = () => {
-      const t = document.documentElement.getAttribute('data-theme') || 'light';
-      el.setAttribute('data-theme', t);
-    };
-
-    syncTheme();
-    const obs = new MutationObserver(syncTheme);
-    obs.observe(document.documentElement, { attributes: true, attributeFilter: ['data-theme'] });
-    return () => {
-      obs.disconnect();
-      el.removeAttribute('data-theme');
-    };
-  }, [fullscreen]);
-
   const zoomIn = () => setScale((prev) => clamp(prev + STEP, MIN, MAX));
   const zoomOut = () => setScale((prev) => clamp(prev - STEP, MIN, MAX));
   const resetZoom = () => setScale(1);