fix(central-ui): rail nav scrolls in its own pane so the footer stays pinned

NavMenu wraps its items in .rail-scroll; site.css fills the sticky rail on
lg+ and lets that wrapper scroll. Kit untouched.

Claude-Session: https://claude.ai/code/session_01Y5ef34iUUMjmwnMpL9xgmB

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Layout/NavMenuTests.cs

@@ -241,4 +241,25 @@ public class NavMenuTests : BunitContext
             Assert.DoesNotContain("/audit/log", cut.Markup);
         });
     }
+
+    /// <summary>
+    /// ThemeShell's <c>nav.side-rail</c> never fills the sticky 100vh rail, so
+    /// a long expanded nav grows past the viewport and pushes
+    /// <c>.rail-foot</c> off-screen. <c>NavMenu</c> wraps its items in a
+    /// <c>div.rail-scroll</c> pane that <c>site.css</c> makes scroll on its
+    /// own (lg+), leaving the brand bar and rail footer pinned. This asserts
+    /// the wrapper exists and actually contains the nav sections, not just
+    /// that the class string appears somewhere in the markup.
+    /// </summary>
+    [Fact]
+    public void NavItems_RenderInsideTheRailScrollPane()
+    {
+        var cut = RenderWithRoles("Administrator", "Designer", "Deployer");
+
+        cut.WaitForAssertion(() =>
+        {
+            Assert.Contains("class=\"rail-scroll\"", cut.Markup);
+            Assert.NotEmpty(cut.FindAll("div.rail-scroll details.rail-section"));
+        });
+    }
 }