feat(frontend): open a live session straight to its pending gate

Opening an in-progress session that has a live Pi runtime now reconnects to
its pending gate instead of the empty landing screen that read as "stopped".
Cold/completed sessions keep the read-only documents panel with its explicit
Resume, so a mere click never spawns a runtime. Backend GET /sessions now
reports a per-session `active` flag (live runtime bound) to drive this.

Also:
- "New session" now closes any open session detail panel (left box).
- The model-activity separator can be dragged to a full 50/50 split
  (was capped at 576px); central-min still guards narrow viewports.

Test fixes uncovered along the way:
- Node 25 ships an experimental global localStorage that shadows jsdom's and
  lacks clear(), failing every jsdom test at setup; install a spec-compliant
  in-memory Storage (feature-detected, inert on CI/LTS).
- Fix 4 pre-existing session-mgmt tests that used an ambiguous getByText for a
  session shown in both nav and header; target the nav item by test id.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -181,6 +181,37 @@ test("opening the panel shows the session documents", async () => {
   expect(await screen.findByText("Domanda originale")).toBeInTheDocument();
 });
 
+test("clicking a session with a live runtime reconnects to its gate instead of the panel", async () => {
+  let resumed: string | null = null;
+  server.use(
+    http.get("http://localhost:8787/sessions", () =>
+      HttpResponse.json([{ ...LIST[0], active: true }])),
+    http.post("http://localhost:8787/sessions/:id/resume", ({ params }) => {
+      resumed = params.id as string;
+      return resumeResult(resumed, true); // warm runtime → alreadyActive
+    }),
+    http.get("http://localhost:8787/sessions/:id", () =>
+      HttpResponse.json({ id: "s1", status: "open", phase: 1 })),
+  );
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno"));
+  await waitFor(() => expect(resumed).toBe("s1"));
+  await waitFor(() => expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s1/events"));
+  // Live view, not the read-only documents panel.
+  expect(screen.queryByText("Domanda originale")).not.toBeInTheDocument();
+});
+
+test("New session closes an open session detail panel", async () => {
+  server.use(http.post("http://localhost:8787/runtime/prewarm", () =>
+    HttpResponse.json({ status: "warming" }, { status: 202 })));
+  wrap();
+  await userEvent.click(await screen.findByText("Attiva uno")); // cold session → panel opens
+  await screen.findByText("Domanda originale");
+  await userEvent.click(screen.getByRole("button", { name: /^new session$/i }));
+  await waitFor(() => expect(screen.queryByText("Domanda originale")).not.toBeInTheDocument());
+  expect(screen.getByText(/type your question/i)).toBeInTheDocument();
+});
+
 test("Archive accordion expands to reveal archived sessions", async () => {
   wrap();
   // archived hidden until the Archive accordion is expanded
@@ -246,7 +277,7 @@ test("an already-active same-session Resume preserves its EventSource and store"
   act(() => first.emitNamed("info", { type: "info", text: "Live state" }, "4"));
   const before = useSessionStore.getState().activityLog.map((entry) => ({ ...entry }));
 
-  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(screen.getByTestId("session-item-s1"));
   await screen.findByText("Domanda originale");
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
 
@@ -278,7 +309,7 @@ test("concurrent same-id Resume invocations share one cold request and replaceme
   const oldSource = FakeEventSource.instances[0];
   act(() => oldSource.emitNamed("info", { type: "info", text: "Old generation" }, "900"));
 
-  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(screen.getByTestId("session-item-s1"));
   const resumeButton = await screen.findByRole("button", { name: /resume/i });
   await userEvent.click(resumeButton);
   await coldStarted.promise;
@@ -343,7 +374,7 @@ test("a committed Resume releases same-id single-flight before its manifest sett
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
   await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F3"));
 
-  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(screen.getByTestId("session-item-s1"));
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
   await waitFor(() => expect(s1ResumeCalls).toBe(2));
   await waitFor(() => expect(useSessionStore.getState().currentPhase).toBe("F4"));
@@ -380,7 +411,7 @@ test("cold same-session Resume keeps the old stream until success then receives
     return resumeResult("s1");
   }));
 
-  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(screen.getByTestId("session-item-s1"));
   await screen.findByText("Domanda originale");
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
   await resumeStarted;
@@ -446,7 +477,7 @@ test("a failed same-session Resume preserves its source, activity, and document
 
   server.use(http.post("http://localhost:8787/sessions/:id/resume", () =>
     new HttpResponse(null, { status: 409 })));
-  await userEvent.click(screen.getByText("Attiva uno"));
+  await userEvent.click(screen.getByTestId("session-item-s1"));
   await userEvent.click(await screen.findByRole("button", { name: /resume/i }));
   await waitFor(() => expect(screen.getByText("Domanda originale")).toBeInTheDocument());