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/test/setup.ts

@@ -1,6 +1,26 @@
 import "@testing-library/jest-dom/vitest";
 import { server } from "./msw";
 
+// Node 25 ships an experimental global `localStorage` that shadows jsdom's Storage and
+// lacks spec methods like clear(), so tests calling localStorage.clear()/setItem() throw.
+// Swap in a spec-compliant in-memory Storage. Feature-detected: inert on runtimes whose
+// localStorage already implements clear() (e.g. CI on an LTS Node with working jsdom).
+if (typeof localStorage === "undefined" || typeof localStorage.clear !== "function") {
+  const store = new Map<string, string>();
+  const storage: Storage = {
+    get length() { return store.size; },
+    clear() { store.clear(); },
+    getItem(key: string) { return store.has(key) ? store.get(key)! : null; },
+    key(index: number) { return Array.from(store.keys())[index] ?? null; },
+    removeItem(key: string) { store.delete(key); },
+    setItem(key: string, value: string) { store.set(String(key), String(value)); },
+  };
+  Object.defineProperty(globalThis, "localStorage", { value: storage, configurable: true, writable: true });
+  if (typeof window !== "undefined") {
+    Object.defineProperty(window, "localStorage", { value: storage, configurable: true, writable: true });
+  }
+}
+
 // jsdom lacks matchMedia; sonner's <Toaster> (mounted in AppShell) needs it.
 if (!window.matchMedia) {
   window.matchMedia = (query: string) =>