Add keyboard shortcuts to switch models

In Assistant, add keyboard shortcuts for switching between models. E.g. Cmd/Ctrl + 1 for ChatGPT 4o, Cmd/Ctrl + 2 for Claude 3.5 Sonnet, etc. This would have the same effect as selecting a model from the model dropdown menu. Ideally, the user can manage their own model assignments, including the ability to assign custom assistants to keyboard shortcuts. This could be from a predefined list of available shortcuts (e.g. Cmd + 1, Cmd + 2, etc.). Alternatively, there could be just one shortcut (e.g. Cmd/Ctrl + Shift + M) to cycle through selected ('favorite') models.