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/useFileActionTerminology.ts

@@ -0,0 +1,22 @@
+import { useTranslation } from 'react-i18next';
+
+/**
+ * File action terminology for web builds
+ * Desktop builds override this with different terminology
+ */
+export function useFileActionTerminology() {
+  const { t } = useTranslation();
+
+  return {
+    uploadFiles: t('fileUpload.uploadFiles', 'Upload Files'),
+    uploadFile: t('fileUpload.uploadFile', 'Upload File'),
+    upload: t('fileUpload.upload', 'Upload'),
+    dropFilesHere: t('fileUpload.dropFilesHere', 'Drop files here or click the upload button'),
+    uploadFromComputer: t('landing.uploadFromComputer', 'Upload from computer'),
+    download: t('download', 'Download'),
+    downloadAll: t('rightRail.downloadAll', 'Download All'),
+    downloadSelected: t('fileManager.downloadSelected', 'Download Selected'),
+    downloadUnavailable: t('downloadUnavailable', 'Download unavailable for this item'),
+    noFilesInStorage: t('fileUpload.noFilesInStorage', 'No files available in storage. Upload some files first.'),
+  };
+}