Update user profile

Update the authenticated user's profile information. Supports both JSON and multipart/form-data requests.

**Authentication Required:** This endpoint requires a valid JWT Bearer token in the Authorization header.

**Profile Fields:**
- `display_name`: User's display name
- `bio`: User biography
- `twitter_handle`: Twitter/X username
- `website`: Personal website URL
- `location`: User location
- `profile_picture`: Profile picture file (multipart/form-data only)

**File Upload:**
- Maximum file size: 5MB
- Allowed types: JPEG, PNG, GIF
- Use multipart/form-data for file uploads