Onboarding: show available models per provider as a selectable list instead of free-text #90

During onboarding, the model name is currently entered as free text.

This is error-prone — users can mistype a model ID, select a model that doesn't exist, or simply not know which models are available for their provider.

Fall back to a free-text field if the model list cannot be fetched (e.g. offline, unsupported provider, or custom base URL).