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/components/split-container.tsx

@@ -135,6 +135,7 @@ interface SplitContainerProps {
   renderPaneEmptyState?: (input: {
     paneId: string;
     paneIsolated: boolean;
+    isFocused: boolean;
     onClosePane: () => void;
   }) => ReactNode;
   focusModeEnabled?: boolean;
@@ -1289,6 +1290,7 @@ function SplitPaneView({
             : (renderPaneEmptyState?.({
                 paneId,
                 paneIsolated,
+                isFocused: isFocused && isWorkspaceFocused,
                 onClosePane: handleClosePane,
               }) ?? null)}
           <SplitDropZone paneId={pane.id} active={showDropZones} preview={dropPreview} />