Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/components/viewer/ScrollAPIBridge.tsx

@@ -0,0 +1,31 @@
+import { useEffect } from 'react';
+import { useScroll } from '@embedpdf/plugin-scroll/react';
+import { useViewer } from '@app/contexts/ViewerContext';
+
+/**
+ * ScrollAPIBridge manages scroll state and exposes scroll actions.
+ * Registers with ViewerContext to provide scroll functionality to UI components.
+ */
+export function ScrollAPIBridge() {
+  const { provides: scroll, state: scrollState } = useScroll();
+  const { registerBridge, triggerImmediateScrollUpdate } = useViewer();
+
+  useEffect(() => {
+    if (scroll && scrollState) {
+      const newState = {
+        currentPage: scrollState.currentPage,
+        totalPages: scrollState.totalPages,
+      };
+      
+      // Trigger immediate update for responsive UI
+      triggerImmediateScrollUpdate(newState.currentPage, newState.totalPages);
+
+      registerBridge('scroll', {
+        state: newState,
+        api: scroll
+      });
+    }
+  }, [scroll, scrollState]);
+
+  return null;
+}