AI Provider visibility improvement

Log in to Kulg web page, go to Settings - AI page.

Now it is not easy to understand that KULG shared AI engine is used by default. Provider combobox show "Google Gemini" selected and only text in API key explains that if key is empty - KULG shared AI will be used.

More visibility will be if "KULG shared AI" selection will be added to the Provider combobox and selected there by default.

If User change Provider to any other (Gemini, OpenAI, etc) then fields API key and Model may be shown or enabled.

In this case understanding of what Provider is used will be much better.