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/useActivityPanelResize.ts

@@ -3,6 +3,7 @@ import type { KeyboardEventHandler, PointerEventHandler, RefObject } from "react
 
 export const ACTIVITY_PANEL_DEFAULT_WIDTH = 384;
 export const ACTIVITY_PANEL_MIN_WIDTH = 288;
+/** Pre-measurement fallback ceiling only; the measured bound is 50% of the container (see below). */
 export const ACTIVITY_PANEL_MAX_WIDTH = 576;
 export const ACTIVITY_CENTRAL_MIN_WIDTH = 512;
 export const ACTIVITY_PANEL_STORAGE_KEY = "thothii:model-activity-panel-width";
@@ -10,11 +11,13 @@ export const ACTIVITY_PANEL_STORAGE_KEY = "thothii:model-activity-panel-width";
 export type ActivityPanelBounds = { min: number; max: number };
 
 export function getActivityPanelBounds(containerWidth: number): ActivityPanelBounds {
+  // The separator may be dragged out to a full 50/50 split. On containers too narrow
+  // for that (below ~1024px) the central-min guard binds first, keeping the chat usable.
   return {
     min: ACTIVITY_PANEL_MIN_WIDTH,
     max: Math.max(
       ACTIVITY_PANEL_MIN_WIDTH,
-      Math.min(ACTIVITY_PANEL_MAX_WIDTH, containerWidth - ACTIVITY_CENTRAL_MIN_WIDTH),
+      Math.min(containerWidth * 0.5, containerWidth - ACTIVITY_CENTRAL_MIN_WIDTH),
     ),
   };
 }