feat(data-app-shell): add collapsible nav rail (navRailHidden)

Add a `navRailHidden` prop to DataAppShell. When true, the desktop
icon nav rail is not rendered, so collapsing the sidebar panel can
hide both the rail and the panel, giving the content full width.

Wired in the Default story to the panel's collapsed state, with a
play-test asserting the rail is hidden when collapsed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: GracePan-Tetra

src/components/composed/DataAppShell/DataAppShell.stories.tsx

@@ -435,6 +435,7 @@ const DefaultShell = ({ initialCollapsed = false }: { initialCollapsed?: boolean
           </Button>
         </>
       }
+      navRailHidden={collapsed}
       sidebarPanel={<WorkflowPanel steps={steps} collapsed={collapsed} onCollapseChange={setCollapsed} />}
     >
       <div className="flex items-center justify-center h-full">
@@ -488,6 +489,12 @@ export const CollapsedWorkflow: Story = {
       expect(canvas.queryByText("Global Filtering")).not.toBeInTheDocument();
       expect(canvas.queryByText("Workflow")).not.toBeInTheDocument();
     });
+
+    await step("Collapsed workflow — app nav rail is hidden", async () => {
+      expect(
+        canvasElement.querySelector("[data-slot='data-app-sidebar-rail']")
+      ).not.toBeInTheDocument();
+    });
   },
   parameters: {
     zephyr: { testCaseId: "SW-T4666" },

src/components/composed/DataAppShell/DataAppShell.tsx

@@ -104,6 +104,8 @@ export interface DataAppShellProps {
   // -- Shell --
   /** Slot rendered between the icon rail and the content (e.g. WorkflowPanel) */
   sidebarPanel?: React.ReactNode;
+  /** Hide the desktop icon nav rail (e.g. when the sidebar panel is collapsed) */
+  navRailHidden?: boolean;
   /** Main content area */
   children: React.ReactNode;
   /** Additional className for the root container */
@@ -492,6 +494,7 @@ function DataAppShell({
   onHelpClick,
   headerActions,
   sidebarPanel,
+  navRailHidden = false,
   children,
   className,
 }: DataAppShellProps) {
@@ -515,8 +518,8 @@ function DataAppShell({
         data-slot="data-app-shell"
         className={cn("flex flex-row w-full h-screen overflow-hidden", className)}
       >
-        {/* Desktop icon rail (hidden on mobile) */}
-        <IconRailSidebar {...sidebarProps} />
+        {/* Desktop icon rail (hidden on mobile, or when navRailHidden) */}
+        {!navRailHidden && <IconRailSidebar {...sidebarProps} />}
 
         {/* Mobile sidebar Sheet */}
         <SheetContent