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/components/fileManager/DragOverlay.tsx

@@ -0,0 +1,44 @@
+import React from 'react';
+import { Stack, Text, useMantineTheme, alpha } from '@mantine/core';
+import UploadFileIcon from '@mui/icons-material/UploadFile';
+import { useTranslation } from 'react-i18next';
+
+interface DragOverlayProps {
+  isVisible: boolean;
+}
+
+const DragOverlay: React.FC<DragOverlayProps> = ({ isVisible }) => {
+  const { t } = useTranslation();
+  const theme = useMantineTheme();
+
+  if (!isVisible) return null;
+
+  return (
+    <div
+      style={{
+        position: 'absolute',
+        top: 0,
+        left: 0,
+        right: 0,
+        bottom: 0,
+        backgroundColor: alpha(theme.colors.blue[6], 0.1),
+        border: `0.125rem dashed ${theme.colors.blue[6]}`,
+        borderRadius: '1.875rem',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        zIndex: 1000,
+        pointerEvents: 'none'
+      }}
+    >
+      <Stack align="center" gap="md">
+        <UploadFileIcon style={{ fontSize: '4rem', color: theme.colors.blue[6] }} />
+        <Text size="xl" fw={500} c="blue.6">
+          {t('fileManager.dropFilesHere', 'Drop files here to upload')}
+        </Text>
+      </Stack>
+    </div>
+  );
+};
+
+export default DragOverlay;