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/RotateAPIBridge.tsx

@@ -0,0 +1,39 @@
+import { useEffect, useState } from 'react';
+import { useRotate } from '@embedpdf/plugin-rotate/react';
+import { useViewer } from '@app/contexts/ViewerContext';
+
+/**
+ * Component that runs inside EmbedPDF context and updates rotation state in ViewerContext
+ */
+export function RotateAPIBridge() {
+  const { provides: rotate, rotation } = useRotate();
+  const { registerBridge } = useViewer();
+  
+  // Store state locally
+  const [_localState, setLocalState] = useState({
+    rotation: 0
+  });
+
+  useEffect(() => {
+    if (rotate) {
+      // Update local state
+      const newState = {
+        rotation
+      };
+      setLocalState(newState);
+
+      // Register this bridge with ViewerContext
+      registerBridge('rotation', {
+        state: newState,
+        api: {
+          rotateForward: () => rotate.rotateForward(),
+          rotateBackward: () => rotate.rotateBackward(),
+          setRotation: (rotationValue: number) => rotate.setRotation(rotationValue),
+          getRotation: () => rotation,
+        }
+      });
+    }
+  }, [rotate, rotation]);
+
+  return null;
+}