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/usePDFProcessor.ts

@@ -0,0 +1,126 @@
+import { useState, useCallback } from 'react';
+import { PDFDocument, PDFPage } from '@app/types/pageEditor';
+import { pdfWorkerManager } from '@app/services/pdfWorkerManager';
+import { createQuickKey } from '@app/types/fileContext';
+
+export function usePDFProcessor() {
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const generatePageThumbnail = useCallback(async (
+    file: File,
+    pageNumber: number,
+    scale: number = 0.5
+  ): Promise<string> => {
+    try {
+      const arrayBuffer = await file.arrayBuffer();
+      const pdf = await pdfWorkerManager.createDocument(arrayBuffer);
+      const page = await pdf.getPage(pageNumber);
+
+      const viewport = page.getViewport({ scale });
+      const canvas = document.createElement('canvas');
+      canvas.width = viewport.width;
+      canvas.height = viewport.height;
+
+      const context = canvas.getContext('2d');
+      if (!context) {
+        throw new Error('Could not get canvas context');
+      }
+
+      await page.render({ canvasContext: context, viewport, canvas }).promise;
+      const thumbnail = canvas.toDataURL();
+
+      // Clean up using worker manager
+      pdfWorkerManager.destroyDocument(pdf);
+
+      return thumbnail;
+    } catch (error) {
+      console.error('Failed to generate thumbnail:', error);
+      throw error;
+    }
+  }, []);
+
+  // Internal function to generate thumbnail from already-opened PDF
+  const generateThumbnailFromPDF = useCallback(async (
+    pdf: any,
+    pageNumber: number,
+    scale: number = 0.5
+  ): Promise<string> => {
+    const page = await pdf.getPage(pageNumber);
+
+    const viewport = page.getViewport({ scale });
+    const canvas = document.createElement('canvas');
+    canvas.width = viewport.width;
+    canvas.height = viewport.height;
+
+    const context = canvas.getContext('2d');
+    if (!context) {
+      throw new Error('Could not get canvas context');
+    }
+
+    await page.render({ canvasContext: context, viewport }).promise;
+    return canvas.toDataURL();
+  }, []);
+
+  const processPDFFile = useCallback(async (file: File): Promise<PDFDocument> => {
+    setLoading(true);
+    setError(null);
+
+    try {
+      const arrayBuffer = await file.arrayBuffer();
+      const pdf = await pdfWorkerManager.createDocument(arrayBuffer);
+      const totalPages = pdf.numPages;
+
+      const pages: PDFPage[] = [];
+
+      // Create pages without thumbnails initially - load them lazily
+      for (let i = 1; i <= totalPages; i++) {
+        pages.push({
+          id: `${createQuickKey(file)}-page-${i}`,
+          pageNumber: i,
+          originalPageNumber: i,
+          thumbnail: null, // Will be loaded lazily
+          rotation: 0,
+          selected: false
+        });
+      }
+
+      // Generate thumbnails for first 10 pages immediately using the same PDF instance
+      const priorityPages = Math.min(10, totalPages);
+      for (let i = 1; i <= priorityPages; i++) {
+        try {
+          const thumbnail = await generateThumbnailFromPDF(pdf, i);
+          pages[i - 1].thumbnail = thumbnail;
+        } catch (error) {
+          console.warn(`Failed to generate thumbnail for page ${i}:`, error);
+        }
+      }
+
+      // Clean up using worker manager
+      pdfWorkerManager.destroyDocument(pdf);
+
+      const document: PDFDocument = {
+        id: `${Date.now()}-${Math.random().toString(36).substr(2, 9)}`,
+        name: file.name,
+        file,
+        pages,
+        totalPages
+      };
+
+      return document;
+    } catch (error) {
+      const errorMessage = error instanceof Error ? error.message : 'Failed to process PDF';
+      setError(errorMessage);
+      throw error;
+    } finally {
+      setLoading(false);
+    }
+  }, [generateThumbnailFromPDF]);
+
+  return {
+    processPDFFile,
+    generatePageThumbnail,
+    loading,
+    error
+  };
+}