Match page width to sso-manager-node; gate Audit nav to admins

- Dashboard, Sessions, and Audit pages now wrap their content in
  <div class="container mt-4">, matching sso-manager-node/proxy's width
  instead of rendering full-bleed inside the fluid shell.
- Audit's nav entry now carries groups: ['admin'] (utils/ui.js), reusing
  the existing synthetic-admin-group nav-gating convention -- the API
  route was already server-side admin-gated, this closes the last gap by
  hiding the nav link/page for non-admins too.
- app-base.js (byte-identical across the 3 apps): added
  app.util.revealItem() and the --sw-content-offset sticky-positioning
  variable, carried over from the same round of changes in
  sso-manager-node/proxy. Not yet called anywhere in this app -- no
  sticky/reveal use case here yet -- but keeps the shared file in sync.

nodejs/public/css/styles.css

@@ -7,6 +7,12 @@ body {
 	display: flex;
 	flex-direction: column;
 	min-height: 100vh;
+	/* Height of the fixed navbar (plus the update banner, while shown --
+	   see top.ejs's showUpdateBanner/dismissUpdateBanner). Lets an in-page
+	   sticky element offset itself below both fixed elements via
+	   `top: var(--sw-content-offset)` instead of colliding with them at the
+	   viewport's true top:0. */
+	--sw-content-offset: 4.5rem;
 }
 
 #spa-shell {