feat(ui): add bottom inset so scrollable content clears the transparent navbar

lib/utils/nav_bar_inset.dart

@@ -0,0 +1,12 @@
+import 'package:flutter/widgets.dart';
+
+/// Bottom inset needed to clear the transparent shell navigation bar.
+///
+/// The shell Scaffold uses `extendBody: true`, so its body (and any route
+/// pushed inside the tab navigators) receives the navbar height plus the system
+/// gesture inset as `MediaQuery.padding.bottom`. Scrollable screens add this as
+/// trailing padding so their last item can scroll clear of the bar while the
+/// content still shows faintly behind it.
+extension NavBarInset on BuildContext {
+  double get navBarBottomInset => MediaQuery.paddingOf(this).bottom;
+}