Add a floating bottom bar style, user-configurable in Settings

Settings > Appearance > "Bottom bar" now switches between the
original edge-to-edge bar and a floating one - inset 16px from both
side edges and clear of the bottom safe area, 28px corners (the same
radius the sheet top/drawer edge use), a 1px line border standing in
for elevation instead of a shadow (product UI stays flat). Persisted
via NooBottomBarStyle, defaulting to the existing attached behavior.

On Android, floating's idle tabs drop their reserved label space -
with no label to leave room for, the icon centers in the whole button
and renders a touch bigger (27 vs 24px) instead of sitting high with a
gap held open under it; the active tab's icon/pill/label position is
unchanged. iOS's tab content is unchanged either way, only the
container.

main.dart sets Scaffold.extendBody while floating so the body scrolls
behind the bar's transparent margin instead of stopping short of it,
matching how the design canvas explored it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/widgets/bottom_nav_bar.dart

@@ -9,6 +9,7 @@ import 'noo/nav/noo_bottom_bar.dart';
 /// drawer's "More" list that isn't one of the pinned [tabs].
 class BottomNavBar extends StatelessWidget {
   final NooNavStyle style;
+  final NooBottomBarStyle barStyle;
   final List<AppTab> tabs;
   final int selectedIndex;
   final ValueChanged<int> onDestinationSelected;
@@ -16,6 +17,7 @@ class BottomNavBar extends StatelessWidget {
   const BottomNavBar({
     super.key,
     required this.style,
+    this.barStyle = NooBottomBarStyle.attached,
     required this.tabs,
     required this.selectedIndex,
     required this.onDestinationSelected,
@@ -25,6 +27,7 @@ class BottomNavBar extends StatelessWidget {
   Widget build(BuildContext context) {
     return NooBottomBar(
       style: style,
+      barStyle: barStyle,
       destinations: [
         for (final tab in tabs)
           NooNavDestination(icon: tab.icon, label: tab.label),