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>

backend/src/routes/sessions.ts

@@ -1,6 +1,6 @@
 import type { FastifyInstance } from "fastify";
 import type { PiProcessManager } from "../pi/pi-process-manager.js";
-import type { ThtRunner } from "../tht/tht-runner.js";
+import type { SessionRow, ThtRunner } from "../tht/tht-runner.js";
 import type { SseHub } from "../sse/sse-hub.js";
 import type { Settings } from "../settings/settings-store.js";
 import { getPrincipal } from "../auth/auth.js";
@@ -213,7 +213,12 @@ export function sessionRoutes(
       const settings = await d.getSettings(principal);
       // Admin RLS is deliberately disabled for a normal 'mine' listing.
       const scopedPrincipal = scope === "mine" ? { ...principal, isAdmin: false } : principal;
-      return await runnerFor(scopedPrincipal).sessionList(settings.workspace);
+      const list: SessionRow[] = await runnerFor(scopedPrincipal).sessionList(settings.workspace);
+      // Annotate each row with whether a live Pi runtime is currently bound. The client
+      // opens an `active` session straight into its live view (reconnecting to its pending
+      // gate), while a cold session keeps its explicit Resume affordance — so a mere click
+      // never spawns a runtime.
+      return list.map((row) => ({ ...row, active: d.mgr.get(row.id) !== undefined }));
     } catch { return storageFailure(reply); }
   });
   app.get("/sessions/:id", async (req, reply) => {