* AI Chat: introduce freemium model concept

* AI Chat: Add mixtral model

* AI Chat: premium's default also becomes leo-expanded

* AI Chat: non-premium users on freemium models get rate limit error which prompts to use a basic (free) model as secondary action

* fix format from chromium upgrade

* AI Chat: introduce "is_freemium_available" boolean feature param

If true, certain freemium models are available to non-premium users. If false, those models are premium-only.

* AI Chat: migrate default model key pref from "chat-default" to "chat-basic"

* AI Chat: reduce claude page content character length to avoid token length mismatch more often

* AI Chat: model menu uses model display name instead of API name