refactor: stop persisting workspace state in the browser

frontend/src/shell/AppShell.new-session.test.tsx

@@ -6,6 +6,7 @@ import { server } from "../test/msw";
 import { FakeEventSource } from "../test/fakeEventSource";
 import { useSessionStore } from "../store/sessionStore";
 import { AppShell } from "./AppShell";
+import { workspacePreferences } from "../workspaces/preferences";
 
 function renderShell() {
   const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
@@ -143,8 +144,7 @@ test("model selector shows the three Pi-enabled models and stores the selected p
     ]);
   });
   await userEvent.selectOptions(select, "qwen3.6-35b-a3b");
-  await waitFor(() => expect(JSON.parse(localStorage.getItem("thothii.workspace-registry.v1.preferences")!))
-    .toEqual({
+  await waitFor(() => expect(workspacePreferences.load()).toEqual({
       workspaceId: "default", provider: "local-qwen", model: "qwen3.6-35b-a3b", thinking: "low",
     }));
 });