fix(app): seed changes tab in explorer pane even when setup tab occupies it

The explorer companion pane retention commit (de9e85862) auto-opens the
setup progress tab into the hidden explorer pane. toggleWorkspaceExplorerPane
only seeded the working_diff (Changes) tab when the pane was completely
empty, so once the setup tab landed there, clicking the explorer toggle
could never surface Changes/Files again via that pane's default tab.

Also fixes two e2e testid locators (workspace-new-tab-menu-trigger) and
one unit test assertion (browser automation handler) that assumed a bare
pane root, which predates that same explorer-pane-retention change: the
workspace root is now always a group with a hidden companion pane, so
unscoped locators/assertions must target the visible/named pane.

packages/app/src/desktop/browser/automation/handler.test.ts

@@ -4,6 +4,7 @@ import { createJSONStorage, type StateStorage } from "zustand/middleware";
 import { mountBrowserAutomationHandler } from "./handler";
 import type { DesktopHostBridge } from "@/desktop/host";
 import { useBrowserStore } from "@/desktop/browser/store";
+import { findPaneById } from "@/stores/workspace-layout-actions";
 import { useWorkspaceLayoutStore } from "@/stores/workspace-layout-store";
 import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
 
@@ -306,11 +307,15 @@ describe("mountBrowserAutomationHandler", () => {
         target: { kind: "browser", browserId: result.browserId },
       }),
     ]);
-    expect(layout?.root).toEqual(
-      expect.objectContaining({
-        kind: "pane",
-        pane: expect.objectContaining({ focusedTabId: previousFocusedTabId }),
-      }),
+    // Workspaces keep a stable hidden explorer companion pane at the root
+    // (see "retain workspace explorer pane"), so the root is a group and the
+    // draft/browser tabs land in the "main" pane, not at the root directly.
+    if (!layout) {
+      throw new Error("Expected workspace layout");
+    }
+    expect(layout.root.kind).toBe("group");
+    expect(findPaneById(layout.root, "main")).toEqual(
+      expect.objectContaining({ focusedTabId: previousFocusedTabId }),
     );
     expect(openedTabs[0]?.tabId).not.toBe(previousFocusedTabId);
     expect(browser.browser.activeWorkspaceBrowsers).toEqual([]);