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/desktop/components/SetupWizard/LoginForm.tsx

@@ -25,7 +25,9 @@ export const LoginForm: React.FC<LoginFormProps> = ({ serverUrl, isSaaS = false,
 
     // Validation
     if (!username.trim()) {
-      setValidationError(t('setup.login.error.emptyUsername', 'Please enter your username'));
+      setValidationError(isSaaS
+        ? t('setup.login.error.emptyEmail', 'Please enter your email')
+        : t('setup.login.error.emptyUsername', 'Please enter your username'));
       return;
     }
 
@@ -132,8 +134,12 @@ export const LoginForm: React.FC<LoginFormProps> = ({ serverUrl, isSaaS = false,
         )}
 
         <TextInput
-          label={t('setup.login.username.label', 'Username')}
-          placeholder={t('setup.login.username.placeholder', 'Enter your username')}
+          label={isSaaS
+            ? t('setup.login.email.label', 'Email')
+            : t('setup.login.username.label', 'Username')}
+          placeholder={isSaaS
+            ? t('setup.login.email.placeholder', 'Enter your email')
+            : t('setup.login.username.placeholder', 'Enter your username')}
           value={username}
           onChange={(e) => {
             setUsername(e.target.value);

frontend/src/desktop/hooks/useFileActionIcons.ts

@@ -0,0 +1,15 @@
+import FolderOpenOutlinedIcon from '@mui/icons-material/FolderOpenOutlined';
+import SaveOutlinedIcon from '@mui/icons-material/SaveOutlined';
+
+/**
+ * File action icons for desktop builds
+ * Overrides core implementation with desktop-appropriate icons
+ */
+export function useFileActionIcons() {
+  return {
+    upload: FolderOpenOutlinedIcon,
+    download: SaveOutlinedIcon,
+    uploadIconName: 'folder-rounded' as const,
+    downloadIconName: 'save-rounded' as const,
+  };
+}

frontend/src/desktop/hooks/useFileActionTerminology.ts

@@ -0,0 +1,22 @@
+import { useTranslation } from 'react-i18next';
+
+/**
+ * File action terminology for desktop builds
+ * Overrides core implementation with desktop-appropriate terminology
+ */
+export function useFileActionTerminology() {
+  const { t } = useTranslation();
+
+  return {
+    uploadFiles: t('fileUpload.openFiles', 'Open Files'),
+    uploadFile: t('fileUpload.openFile', 'Open File'),
+    upload: t('fileUpload.open', 'Open'),
+    dropFilesHere: t('fileUpload.dropFilesHereOpen', 'Drop files here or click the open button'),
+    uploadFromComputer: t('landing.openFromComputer', 'Open from computer'),
+    download: t('save', 'Save'),
+    downloadAll: t('rightRail.saveAll', 'Save All'),
+    downloadSelected: t('fileManager.saveSelected', 'Save Selected'),
+    downloadUnavailable: t('saveUnavailable', 'Save unavailable for this item'),
+    noFilesInStorage: t('fileUpload.noFilesInStorageOpen', 'No files available in storage. Open some files first.'),
+  };
+}