Refactor ProfileForm to support draft state management and improve file handling

- Introduced functionality to save and restore form state as a draft when creating a new voice profile.
- Added helper functions for converting files to and from base64 format to facilitate file handling.
- Updated the API types to use a more flexible LanguageCode type for language parameters.
- Enhanced the UI store to manage profile form drafts, improving user experience during profile creation.

app/src/lib/hooks/useTranscription.ts

@@ -1,9 +1,10 @@
 import { useMutation } from '@tanstack/react-query';
 import { apiClient } from '@/lib/api/client';
+import type { LanguageCode } from '@/lib/constants/languages';
 
 export function useTranscription() {
   return useMutation({
-    mutationFn: ({ file, language }: { file: File; language?: 'en' | 'zh' }) =>
+    mutationFn: ({ file, language }: { file: File; language?: LanguageCode }) =>
       apiClient.transcribeAudio(file, language),
   });
 }