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.tsx

@@ -122,7 +122,17 @@ export function AppShell() {
 
   function openPanel(id: string) {
     const s = sessions.find((x) => x.id === id);
-    if (s) { setPanelSession(s); setShowActivity(false); }
+    if (!s) return;
+    // A session with a live Pi runtime opens straight into its live view: doResume
+    // reconnects to the already-active runtime and replays its pending gate, so an
+    // in-progress session never shows an empty screen that reads as "stopped". Cold or
+    // completed sessions keep the read-only documents panel (with its explicit Resume),
+    // so a mere click never spawns a runtime.
+    if (s.active && s.status === "open" && !s.archived && !isForeignSession(s)) {
+      void doResume(id);
+      return;
+    }
+    setPanelSession(s); setShowActivity(false);
   }
   function toggleActivity() {
     setShowActivity((v) => {
@@ -351,6 +361,9 @@ export function AppShell() {
   function startNewSession() {
     invalidateResumeIntent();
     resetSession();
+    // Starting a new question closes any open session detail panel: the reader is
+    // moving away from that session, so its left-hand box must not linger.
+    setPanelSession(null);
     setAwaitingQuestion(true);
     setCreatingSession(false);
     selectActiveSession(null);