feat(image-viewer): integrate ImageViewer component and update FolderPicker layout

- Added ImageViewer component to the App for displaying processed images.
- Updated FolderPicker layout for improved user guidance and aesthetics.
- Refactored ShortcutItem and Shortcuts components for better structure and clarity.
- Introduced ItemModal component for future use.

frontend/src/components/item-modal/ItemModal.tsx

@@ -0,0 +1,7 @@
+export const ItemModal = () => {
+    return (
+        <div class="fixed inset-4 bg-white border border-neutral-300">
+            ItemModal
+        </div>
+    );
+};