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

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Layout/NavMenu.razor

@@ -5,8 +5,12 @@
    (brand bar + responsive hamburger) belongs to ThemeShell; this component
    contributes only the nav items. Collapsible sections use the kit's
    NavRailSection (<details>); their open/closed state is persisted client-side
-   by the kit's nav-state.js (localStorage, keyed by Key) — no JS interop here. *@
+   by the kit's nav-state.js (localStorage, keyed by Key) — no JS interop here.
+   Everything below is wrapped in .rail-scroll, the scroll pane defined in
+   site.css: a long nav scrolls under the brand bar while the rail footer
+   stays pinned, and the kit itself is untouched. *@
 
+<div class="rail-scroll">
 <NavRailItem Href="/" Text="Dashboard" Match="NavLinkMatch.All" />
 
 <AuthorizeView>
@@ -153,3 +157,4 @@
         </AuthorizeView>
     </Authorized>
 </AuthorizeView>
+</div>