fix: correct avatar upload and DiceBear URL version

- Add api.upload() to client.ts that passes FormData without setting
  Content-Type, letting the browser set multipart/form-data with the
  correct boundary (was causing 422 on the upload endpoint)
- Use api.upload() instead of api.post() for avatar file upload
- Update DiceBear URLs from v6 to 9.x in both frontend and backend

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/src/rehearsalhub/services/avatar.py

@@ -9,7 +9,7 @@ class AvatarService:
     """Service for generating and managing user avatars."""
 
     def __init__(self):
-        self.base_url = "https://api.dicebear.com/v6"
+        self.base_url = "https://api.dicebear.com/9.x"
 
     async def generate_avatar_url(self, seed: str, style: str = "identicon") -> str:
         """Generate a DiceBear avatar URL for the given seed.