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/hooks/useFileWithUrl.ts

@@ -0,0 +1,51 @@
+import { useMemo } from 'react';
+import { isFileObject } from '@app/types/fileContext';
+
+/**
+ * Hook to convert a File object to { file: File; url: string } format
+ * Creates blob URL on-demand and handles cleanup
+ */
+export function useFileWithUrl(file: File | Blob | null): { file: File | Blob; url: string } | null {
+  return useMemo(() => {
+    if (!file) return null;
+
+    // Validate that file is a proper File, StirlingFile, or Blob object
+    if (!isFileObject(file) && !(file instanceof Blob)) {
+      console.warn('useFileWithUrl: Expected File or Blob, got:', file);
+      return null;
+    }
+
+    try {
+      const url = URL.createObjectURL(file);
+
+      // Return object with cleanup function
+      const result = { file, url };
+
+      // Store cleanup function for later use
+      (result as any)._cleanup = () => URL.revokeObjectURL(url);
+
+      return result;
+    } catch (error) {
+      console.error('useFileWithUrl: Failed to create object URL:', error, file);
+      return null;
+    }
+  }, [file]);
+}
+
+/**
+ * Hook variant that returns cleanup function separately
+ */
+export function useFileWithUrlAndCleanup(file: File | null): {
+  fileObj: { file: File; url: string } | null;
+  cleanup: () => void;
+} {
+  return useMemo(() => {
+    if (!file) return { fileObj: null, cleanup: () => {} };
+
+    const url = URL.createObjectURL(file);
+    const fileObj = { file, url };
+    const cleanup = () => URL.revokeObjectURL(url);
+
+    return { fileObj, cleanup };
+  }, [file]);
+}