feat(frontend): sessions list/create + steering + resume + selectors

- NavSessions: left nav listing sessions via listSessions (TanStack Query),
  clicking resumes via resumeSession and sets active; active item highlighted
- NewSessionDialog: ShadCn dialog with question textarea, workspace native
  select (from listWorkspaces), graceful model field (dropdown when models
  list is non-empty, free-text input when empty per §9 degradation),
  optional thinking/provider fields; try/catch surfaces errors inline
- SteerInput: text input + button POSTs to postSteer, clears on send,
  supports Enter key; only shown when a session is active
- WorkflowBar: renders 8 phases (F1..F8) with data-active highlighting
  driven by useSessionStore.currentPhase (new store field + setPhase action)
- AppShell: wires all four components; right artifact sidebar now collapsible
  via a toggle button (useState sidebarOpen); replaces old "Nuova domanda"
  button with NewSessionDialog trigger
- f1-loop.test: updated to drive through the new dialog flow (open → fill
  question → submit → wait for SSE → respond)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/shell/SteerInput.test.tsx

@@ -0,0 +1,69 @@
+// frontend/src/shell/SteerInput.test.tsx
+import { render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { http, HttpResponse } from "msw";
+import { server } from "../test/msw";
+import { SteerInput } from "./SteerInput";
+
+beforeEach(() => {
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/steer", () =>
+      new HttpResponse(null, { status: 204 }),
+    ),
+  );
+});
+
+test("renders a text input and submit button", () => {
+  render(<SteerInput sessionId="s1" />);
+  expect(screen.getByRole("textbox")).toBeInTheDocument();
+  expect(screen.getByRole("button", { name: /invia|send|steer/i })).toBeInTheDocument();
+});
+
+test("submitting typed text POSTs to /steer and clears the input", async () => {
+  let captured: unknown = null;
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/steer", async ({ request, params }) => {
+      captured = { id: params.id, body: await request.json() };
+      return new HttpResponse(null, { status: 204 });
+    }),
+  );
+  render(<SteerInput sessionId="s1" />);
+  const input = screen.getByRole("textbox");
+  await userEvent.type(input, "redirect to plan B");
+  await userEvent.click(screen.getByRole("button", { name: /invia|send|steer/i }));
+  await waitFor(() => {
+    expect(captured).toEqual({ id: "s1", body: { text: "redirect to plan B" } });
+  });
+  expect(input).toHaveValue("");
+});
+
+test("pressing Enter in the input submits the steer", async () => {
+  let captured: unknown = null;
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/steer", async ({ request }) => {
+      captured = await request.json();
+      return new HttpResponse(null, { status: 204 });
+    }),
+  );
+  render(<SteerInput sessionId="s1" />);
+  const input = screen.getByRole("textbox");
+  await userEvent.type(input, "use approach X{Enter}");
+  await waitFor(() => {
+    expect(captured).toEqual({ text: "use approach X" });
+  });
+});
+
+test("does not POST when input is empty", async () => {
+  let called = false;
+  server.use(
+    http.post("http://localhost:8787/sessions/:id/steer", () => {
+      called = true;
+      return new HttpResponse(null, { status: 204 });
+    }),
+  );
+  render(<SteerInput sessionId="s1" />);
+  await userEvent.click(screen.getByRole("button", { name: /invia|send|steer/i }));
+  // small wait to ensure the call didn't go through
+  await new Promise((r) => setTimeout(r, 50));
+  expect(called).toBe(false);
+});