fix: improve error handling for avatar uploads

- Change invalid file type error from 400 to 422 for better frontend handling
- Add specific error message for 422 responses in frontend
- Improve error message clarity
- Better error classification and user guidance

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

api/src/rehearsalhub/routers/auth.py

@@ -87,8 +87,8 @@ async def upload_avatar(
     if not file.content_type.startswith("image/"):
         print("Invalid file type")
         raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST,
-            detail="Only image files are allowed"
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail="Only image files are allowed (JPG, PNG, GIF, etc.)"
         )
     
     # Validate file size (5MB limit for upload endpoint)