Feature Proposal: Prioritize configured providers in model selection dialog

[sgtsquiggs]

Suggest changing the model selection dialog to prioritize configured providers over unconfigured ones. Providers you've set up (e.g., added API keys for) now appear first in the list, making it faster to find and switch between your active providers.

Sorting logic:

1. "Charm Hyper" always first (if configured)
2. Configured providers (sorted by original order)
3. Unconfigured providers (sorted by original order)

This is a small UX improvement that reduces scrolling when you have many providers available but only a few actively configured.

Working here:
internal/ui/dialog/models.go
internal/ui/dialog/models_test.go