Expand workspace panel actions and keyboard navigation (#3685)

* fix(app): keep file navigation in place

Retargeting a File tab allocated a new tab identity, causing the tab track to unmount and remount during navigation. Preserve the identity for File-to-File changes while retaining new instances for cross-kind replacements.

* feat(app): expand workspace panel actions

Keep the Command Center aligned with workspace tab and pane primitives. Add explicit panel placement, focused open behavior, keyboard navigation for empty panes, and a Changes pill that toggles the visible Side panel without closing its tab.

packages/app/src/workspace-tabs/side-panel.test.ts

@@ -31,6 +31,7 @@ import {
   openSupportingTab,
   showSidePanel,
   toggleSidePanel,
+  toggleSupportingTab,
 } from "@/workspace-tabs/side-panel";
 
 const WORKSPACE_KEY = "server-1:ws-main";
@@ -205,6 +206,27 @@ describe("non-compact with splits", () => {
     expect(isSidePanelOpen(wide)).toBe(false);
     expect(tabKinds()).toEqual(["files"]);
   });
+
+  it("toggles the visible supporting target without closing its tab", () => {
+    const changes = {
+      ...wide,
+      target: { kind: "working_diff" } as const,
+      openInSidePanelByDefault: true,
+    };
+    const tabId = toggleSupportingTab(changes);
+    openSidePanelView({ ...wide, view: "files" });
+
+    toggleSupportingTab(changes);
+    expect(isSidePanelOpen(wide)).toBe(true);
+    expect(focusedPaneTabKinds()).toContain("working_diff");
+
+    toggleSupportingTab(changes);
+    expect(isSidePanelOpen(wide)).toBe(false);
+    expect(tabKinds()).toContain("working_diff");
+
+    expect(toggleSupportingTab(changes)).toBe(tabId);
+    expect(isSidePanelOpen(wide)).toBe(true);
+  });
 });
 
 describe("non-compact without splits", () => {