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/useIndexedDBThumbnail.ts

@@ -0,0 +1,83 @@
+import { useState, useEffect } from "react";
+import { StirlingFileStub } from "@app/types/fileContext";
+import { useIndexedDB } from "@app/contexts/IndexedDBContext";
+import { generateThumbnailForFile } from "@app/utils/thumbnailUtils";
+import { FileId } from "@app/types/fileContext";
+
+
+/**
+ * Hook for IndexedDB-aware thumbnail loading
+ * Handles thumbnail generation for files not in IndexedDB
+ */
+export function useIndexedDBThumbnail(file: StirlingFileStub | undefined | null): {
+  thumbnail: string | null;
+  isGenerating: boolean
+} {
+  const [thumb, setThumb] = useState<string | null>(null);
+  const [generating, setGenerating] = useState(false);
+  const indexedDB = useIndexedDB();
+
+  useEffect(() => {
+    let cancelled = false;
+
+    async function loadThumbnail() {
+      if (!file) {
+        setThumb(null);
+        return;
+      }
+
+      // First priority: use stored thumbnail
+      if (file.thumbnailUrl) {
+        setThumb(file.thumbnailUrl);
+        return;
+      }
+
+      // Second priority: generate thumbnail for files under 100MB
+      if (file.size < 100 * 1024 * 1024 && !generating) {
+        setGenerating(true);
+        try {
+          let fileObject: File;
+
+          // Try to load file from IndexedDB using new context
+          if (file.id && indexedDB) {
+            const loadedFile = await indexedDB.loadFile(file.id as FileId);
+            if (!loadedFile) {
+              throw new Error('File not found in IndexedDB');
+            }
+            fileObject = loadedFile;
+          } else {
+            throw new Error('File ID not available or IndexedDB context not available');
+          }
+
+          // Use the universal thumbnail generator
+          const thumbnail = await generateThumbnailForFile(fileObject);
+          if (!cancelled) {
+            setThumb(thumbnail);
+
+            // Save thumbnail to IndexedDB for persistence
+            if (file.id && indexedDB && thumbnail) {
+              try {
+                await indexedDB.updateThumbnail(file.id as FileId, thumbnail);
+              } catch (error) {
+                console.warn('Failed to save thumbnail to IndexedDB:', error);
+              }
+            }
+          }
+        } catch (error) {
+          console.warn('Failed to generate thumbnail for file', file.name, error);
+          if (!cancelled) setThumb(null);
+        } finally {
+          if (!cancelled) setGenerating(false);
+        }
+      } else {
+        // Large files - no thumbnail
+        setThumb(null);
+      }
+    }
+
+    loadThumbnail();
+    return () => { cancelled = true; };
+  }, [file, file?.thumbnailUrl, file?.id, indexedDB, generating]);
+
+  return { thumbnail: thumb, isGenerating: generating };
+}