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/ThumbnailAPIBridge.tsx

@@ -0,0 +1,23 @@
+import { useEffect } from 'react';
+import { useThumbnailCapability } from '@embedpdf/plugin-thumbnail/react';
+import { useViewer } from '@app/contexts/ViewerContext';
+
+/**
+ * ThumbnailAPIBridge provides thumbnail generation functionality.
+ * Exposes thumbnail API to UI components without managing state.
+ */
+export function ThumbnailAPIBridge() {
+  const { provides: thumbnail } = useThumbnailCapability();
+  const { registerBridge } = useViewer();
+
+  useEffect(() => {
+    if (thumbnail) {
+      registerBridge('thumbnail', {
+        state: null, // No state - just provides API
+        api: thumbnail
+      });
+    }
+  }, [thumbnail]);
+
+  return null;
+}