fix(portal-admin): html/body overflow-y hidden as a shell shield

The admin shell is a "fills 100vh, never scrolls the body" layout —
<app-root> is height: 100vh and <main> owns its own overflow-y. If
anything ever pushes content past the viewport (a chart sizing
escape, a future iframe, a flex bug), we'd rather clip at the root
than show a phantom body scrollbar plus the empty space under the
footer that comes with it.

apps/portal-admin/src/styles.css

@@ -16,3 +16,18 @@
 
 /* Brand palette tokens — shared with portal-shell. */
 @import '../../../libs/shared/tokens/src/brand-tokens.css';
+
+/*
+ * The admin shell is a "fills the viewport, never scrolls the body"
+ * layout: <app-root> is locked at height: 100vh, and <main> owns its
+ * own overflow-y. If anything inside the shell ever pushes content
+ * past the viewport (a wide chart, a flex sizing bug, a third-party
+ * iframe), we'd rather clip than show a phantom body scrollbar plus
+ * the empty space below the footer that comes with it. The element-
+ * level overflow on <main> still produces a real, scrollable region
+ * for content that overflows by design.
+ */
+html,
+body {
+  overflow-y: hidden;
+}