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/contexts/FilesModalContext.tsx

@@ -0,0 +1,115 @@
+import React, { createContext, useContext, useState, useCallback, useMemo } from 'react';
+import { useFileHandler } from '@app/hooks/useFileHandler';
+import { useFileActions } from '@app/contexts/FileContext';
+import { StirlingFileStub } from '@app/types/fileContext';
+import { fileStorage } from '@app/services/fileStorage';
+
+interface FilesModalContextType {
+  isFilesModalOpen: boolean;
+  openFilesModal: (options?: { insertAfterPage?: number; customHandler?: (files: File[], insertAfterPage?: number) => void }) => void;
+  closeFilesModal: () => void;
+  onFileUpload: (files: File[]) => void;
+  onRecentFileSelect: (stirlingFileStubs: StirlingFileStub[]) => void;
+  onModalClose?: () => void;
+  setOnModalClose: (callback: () => void) => void;
+}
+
+const FilesModalContext = createContext<FilesModalContextType | null>(null);
+
+export const FilesModalProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
+  const { addFiles } = useFileHandler();
+  const { actions } = useFileActions();
+  const [isFilesModalOpen, setIsFilesModalOpen] = useState(false);
+  const [onModalClose, setOnModalClose] = useState<(() => void) | undefined>();
+  const [insertAfterPage, setInsertAfterPage] = useState<number | undefined>();
+  const [customHandler, setCustomHandler] = useState<((files: File[], insertAfterPage?: number) => void) | undefined>();
+
+  const openFilesModal = useCallback((options?: { insertAfterPage?: number; customHandler?: (files: File[], insertAfterPage?: number) => void }) => {
+    setInsertAfterPage(options?.insertAfterPage);
+    setCustomHandler(() => options?.customHandler);
+    setIsFilesModalOpen(true);
+  }, []);
+
+  const closeFilesModal = useCallback(() => {
+    setIsFilesModalOpen(false);
+    setInsertAfterPage(undefined); // Clear insertion position
+    setCustomHandler(undefined); // Clear custom handler
+    onModalClose?.();
+  }, [onModalClose]);
+
+  const handleFileUpload = useCallback((files: File[]) => {
+    if (customHandler) {
+      // Use custom handler for special cases (like page insertion)
+      customHandler(files, insertAfterPage);
+    } else {
+      // Use normal file handling
+      addFiles(files);
+    }
+    closeFilesModal();
+  }, [addFiles, closeFilesModal, insertAfterPage, customHandler]);
+
+  const handleRecentFileSelect = useCallback(async (stirlingFileStubs: StirlingFileStub[]) => {
+    if (customHandler) {
+      // Load the actual files from storage for custom handler
+      try {
+        const loadedFiles: File[] = [];
+        for (const stub of stirlingFileStubs) {
+          const stirlingFile = await fileStorage.getStirlingFile(stub.id);
+          if (stirlingFile) {
+            loadedFiles.push(stirlingFile);
+          }
+        }
+        
+        if (loadedFiles.length > 0) {
+          customHandler(loadedFiles, insertAfterPage);
+        }
+      } catch (error) {
+        console.error('Failed to load files for custom handler:', error);
+      }
+    } else {
+      // Normal case - use addStirlingFileStubs to preserve metadata
+      if (actions.addStirlingFileStubs) {
+        actions.addStirlingFileStubs(stirlingFileStubs, { selectFiles: true });
+      } else {
+        console.error('addStirlingFileStubs action not available');
+      }
+    }
+    closeFilesModal();
+  }, [actions.addStirlingFileStubs, closeFilesModal, customHandler, insertAfterPage]);
+
+  const setModalCloseCallback = useCallback((callback: () => void) => {
+    setOnModalClose(() => callback);
+  }, []);
+
+  const contextValue: FilesModalContextType = useMemo(() => ({
+    isFilesModalOpen,
+    openFilesModal,
+    closeFilesModal,
+    onFileUpload: handleFileUpload,
+    onRecentFileSelect: handleRecentFileSelect,
+    onModalClose,
+    setOnModalClose: setModalCloseCallback,
+  }), [
+    isFilesModalOpen,
+    openFilesModal,
+    closeFilesModal,
+    handleFileUpload,
+    handleRecentFileSelect,
+    onModalClose,
+    setModalCloseCallback,
+  ]);
+
+  return (
+    <FilesModalContext.Provider value={contextValue}>
+      {children}
+    </FilesModalContext.Provider>
+  );
+};
+
+export const useFilesModalContext = () => {
+  const context = useContext(FilesModalContext);
+  if (!context) {
+    throw new Error('useFilesModalContext must be used within FilesModalProvider');
+  }
+  return context;
+};