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/PanAPIBridge.tsx

@@ -0,0 +1,45 @@
+import { useEffect, useState } from 'react';
+import { usePan } from '@embedpdf/plugin-pan/react';
+import { useViewer } from '@app/contexts/ViewerContext';
+
+/**
+ * Component that runs inside EmbedPDF context and updates pan state in ViewerContext
+ */
+export function PanAPIBridge() {
+  const { provides: pan, isPanning } = usePan();
+  const { registerBridge } = useViewer();
+  
+  // Store state locally
+  const [_localState, setLocalState] = useState({
+    isPanning: false
+  });
+
+  useEffect(() => {
+    if (pan) {
+      // Update local state
+      const newState = {
+        isPanning
+      };
+      setLocalState(newState);
+
+      // Register this bridge with ViewerContext
+      registerBridge('pan', {
+        state: newState,
+        api: {
+          enable: () => {
+            pan.enablePan();
+          },
+          disable: () => {
+            pan.disablePan();
+          },
+          toggle: () => {
+            pan.togglePan();
+          },
+          makePanDefault: () => pan.makePanDefault(),
+        }
+      });
+    }
+  }, [pan, isPanning]);
+
+  return null;
+}