refactor: stop persisting workspace state in the browser

frontend/src/shell/NewSessionDialog.test.tsx

@@ -6,6 +6,7 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { server } from "../test/msw";
 import { canonicalWorkspaceFixture, workspaceRevisionFixture, workspaceSummaryFixture } from "../test/workspace-fixtures";
 import { NewSessionDialog } from "./NewSessionDialog";
+import { workspacePreferences } from "../workspaces/preferences";
 
 function renderDialog() {
   const onCreated = vi.fn();
@@ -28,7 +29,7 @@ test("the form has only a question field (no workspace/model/provider/thinking)"
   expect(screen.queryByLabelText(/thinking/i)).not.toBeInTheDocument();
 });
 
-test("submitting includes browser-local migrated preferences and calls onCreated", async () => {
+test("submitting includes ephemeral migrated preferences and calls onCreated", async () => {
   localStorage.clear();
   let body: unknown = null;
   server.use(
@@ -71,9 +72,9 @@ test("first-run direct dialog creation waits for registry policy without a mount
   const revision = {
     id: "psd-clinical", commit: "a".repeat(40), blob: "b".repeat(40), snapshotPath: "/snapshot",
   };
-  localStorage.setItem("thothii.workspace-registry.v1.preferences", JSON.stringify({
+  workspacePreferences.save({
     workspaceId: "psd-clinical", provider: "deepseek", model: "deepseek-v4-pro", thinking: "medium",
-  }));
+  });
   server.use(
     http.get("/api/workspaces", () => {
       summaryRequestStarted = true;