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/wwwroot/css/site.css

@@ -170,3 +170,24 @@
     /* Backdrop — slightly darker overlay in dark mode */
     --sb-backdrop: rgba(0, 0, 0, 0.6);
 }
+
+/* ── Rail nav scroll pane ────────────────────────────────────────────────
+   ThemeShell's nav.side-rail is a flex column inside the sticky 100vh
+   #theme-rail (lg+), but it never fills that height and nothing inside it
+   scrolls, so a long nav grows past the viewport and carries the footer
+   with it. Fill the rail and let the nav items (NavMenu's .rail-scroll
+   wrapper) scroll while the brand bar and .rail-foot stay put. Below lg the
+   rail is inline and scrolls with the page, so the rule is scoped to lg+.
+   The kit is untouched (deferred-work register: upstream into
+   ZB.MOM.WW.Theme so the other consumers get it too). */
+@media (min-width: 992px) {
+    #theme-rail > .side-rail {
+        height: 100%;
+    }
+    .rail-scroll {
+        flex: 1 1 auto;
+        min-height: 0;
+        overflow-y: auto;
+        overscroll-behavior: contain;
+    }
+}