Drag to reorder provider instances in the model selector

[deacon-botdoctor]

The provider rail in the model selector currently uses a fixed provider order. With several native and ACP providers, frequently used harnesses can end up below providers used less often.

Please let users press and drag a provider icon up or down directly in the model selector, then save that order across closing the menu and restarting T3.

For example, my preferred order is Hermes, Oh My Pi, Codex, Claude, and Grok. A normal click should still select the provider. A small drag threshold should prevent accidental rearrangement. Favorites can remain pinned at the top, and keyboard users should have an equivalent reorder action. Newly added providers should appear after the saved ones, and temporarily unavailable providers should retain their position.

This concerns provider instances in the selector rail. It differs from #19082, which requests arbitrary ordering of individual favorite models across providers.

We plan to implement a local customization for this workflow. This discussion requests upstream consideration; it does not assume maintainer approval of an upstream implementation.

Submitted with Codex at the user's request.