Various fixes for desktop (#4976)

# Description of Changes
- ~Force classic logo~
- Refer to email instead of username in SaaS sign in flow
- Allow drag-and-drop files into desktop app
- Convert terminology & icons from upload/download to open/save in
desktop version

frontend/src/core/hooks/useFileActionIcons.ts

@@ -0,0 +1,15 @@
+import UploadIcon from '@mui/icons-material/Upload';
+import DownloadOutlinedIcon from '@mui/icons-material/DownloadOutlined';
+
+/**
+ * File action icons for web builds
+ * Desktop builds override this with different icons
+ */
+export function useFileActionIcons() {
+  return {
+    upload: UploadIcon,
+    download: DownloadOutlinedIcon,
+    uploadIconName: 'upload' as const,
+    downloadIconName: 'download' as const,
+  };
+}