fix: use user-configured OCR model and attribute OCR errors correctly (#576)

Previously, the OCR model selection was inverted: users with a license key
always used Chatbox AI OCR even when they had configured a custom OCR model.
Now user-configured OCR models take priority, with Chatbox AI as fallback.

Also fixes misleading error messages when OCR fails — errors now correctly
identify the OCR provider instead of blaming the chat model's provider.