Per-chat backends

**What problem would this solve?**
Currently in the Goose desktop app, the backend (either local or ACP-compatible remote) is globally applied to all of the chats in a window. This makes it difficult to multitask when you want to connect to different servers or have a mix of remote and local backends in simultaneous chat sessions. Ostensibly this can be achieved by configuring the setting per window but (a) that's annoying / not ergonomic because it forces you to rely on window management to jump between chats rather than the built-in chat management and (b) in my limited testing it didn't work -- the second window wouldn't load chats.

**What would a good outcome look like?**
The backend would be configurable per chat rather than per window, the way that model and directory are set per chat. This is how the Claude desktop app functions fwiw.

**Possible approaches**
I'm not familiar with the codebase but here's some initial direction from an agent:

> Today the backend is global because it's the ACP connection itself: the desktop picks one backend per window (local goose serve or the single external host), and every session in that window talks over one shared connection. Model and directory are already per-chat because they're session-scoped state on the server.

> The approach is to make the backend a per-chat routing attribute: model backends as first-class profiles, give each one its own ACP connection (replacing the current singleton), and scope all session operations to a backendId. The UI selector is the easy part — the real work is threading backend id through every session call and handling per-connection recovery and notification routing.

**Additional context**
N/A

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).