fix: web recorder ignores selected transcription language, always sends multi (#5547)

## Cause

`GET /v1/users/transcription-preferences` never returned the `language`
field, so `prefs.language` was always `undefined` in the recorder. This
caused the WebSocket to fall back to `language=multi` (auto-detect),
overriding whatever language the user had set in Settings.

## Fix

- Added `language` field to `TranscriptionPreferencesResponse` in
`backend/routers/users.py`
- Updated `get_user_transcription_preferences` in
`backend/database/users.py` to read and return the top-level `language`
field from the user document

## Demo

**Before** 


https://github.com/user-attachments/assets/2f66ab73-e674-4255-9603-4acc33d3b777

**After** 



https://github.com/user-attachments/assets/594a4b05-9559-4d8d-a67f-42c395b4f5e3


closes #4808

Author: mdmohsin7

backend/database/users.py

@@ -997,7 +997,7 @@ def get_user_transcription_preferences(uid: str) -> dict:
     Get the user's transcription preferences.
 
     Returns:
-        dict with 'single_language_mode' (bool) and 'vocabulary' (List[str])
+        dict with 'single_language_mode' (bool), 'vocabulary' (List[str]), and 'language' (str)
     """
     user_ref = db.collection('users').document(uid)
     user_doc = user_ref.get()
@@ -1008,9 +1008,10 @@ def get_user_transcription_preferences(uid: str) -> dict:
         return {
             'single_language_mode': prefs.get('single_language_mode', False),
             'vocabulary': prefs.get('vocabulary', []),
+            'language': user_data.get('language', ''),
         }
 
-    return {'single_language_mode': False, 'vocabulary': []}
+    return {'single_language_mode': False, 'vocabulary': [], 'language': ''}
 
 
 def get_agent_vm(uid: str) -> Optional[dict]:

backend/routers/users.py

@@ -498,6 +498,7 @@ def set_user_language(data: dict, uid: str = Depends(auth.get_current_user_uid))
 class TranscriptionPreferencesResponse(BaseModel):
     single_language_mode: bool = False
     vocabulary: List[str] = []
+    language: str = ''
 
 
 class TranscriptionPreferencesUpdate(BaseModel):