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/SpreadAPIBridge.tsx

@@ -0,0 +1,39 @@
+import { useEffect } from 'react';
+import { useSpread, SpreadMode } from '@embedpdf/plugin-spread/react';
+import { useViewer } from '@app/contexts/ViewerContext';
+
+/**
+ * Component that runs inside EmbedPDF context and updates spread state in ViewerContext
+ */
+export function SpreadAPIBridge() {
+  const { provides: spread, spreadMode } = useSpread();
+  const { registerBridge } = useViewer();
+
+  useEffect(() => {
+    if (spread) {
+      const newState = {
+        spreadMode, 
+        isDualPage: spreadMode !== SpreadMode.None
+      };
+
+      // Register this bridge with ViewerContext
+      registerBridge('spread', {
+        state: newState,
+        api: {
+          setSpreadMode: (mode: SpreadMode) => {
+            spread.setSpreadMode(mode);
+          },
+          getSpreadMode: () => spread.getSpreadMode(),
+          toggleSpreadMode: () => {
+            // Toggle between None and Odd (most common dual-page mode)
+            const newMode = spreadMode === SpreadMode.None ? SpreadMode.Odd : SpreadMode.None;
+            spread.setSpreadMode(newMode);
+          },
+          SpreadMode: SpreadMode, // Export enum for reference
+        }
+      });
+    }
+  }, [spread, spreadMode]);
+
+  return null;
+}