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

portal-shell's app.scss has the same "100vh shell, internal main
scroll" shape as portal-admin. Mirroring the shield keeps the two
apps consistent — any future layout escape stops at the shell
boundary instead of producing a phantom body scrollbar.

apps/portal-shell/src/styles.css

@@ -20,3 +20,18 @@
 
 /* Brand palette tokens — shared with `portal-admin` once it ships. */
 @import '../../../libs/shared/tokens/src/brand-tokens.css';
+
+/*
+ * The 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. Same shield as portal-admin.
+ */
+html,
+body {
+  overflow-y: hidden;
+}