feat: add swipe navigation between main tabs

lib/screens/home_shell.dart

@@ -12,6 +12,8 @@ class HomeShell extends StatefulWidget {
 }
 
 class _HomeShellState extends State<HomeShell> {
+  static const double _swipeVelocityThreshold = 300;
+
   int _currentIndex = 0;
   final _collectionsKey = GlobalKey<CollectionsScreenState>();
   final _scanKey = GlobalKey<ScanTabState>();
@@ -26,12 +28,30 @@ class _HomeShellState extends State<HomeShell> {
     setState(() => _currentIndex = i);
   }
 
+  void _handleHorizontalSwipe(DragEndDetails details) {
+    final velocity = details.primaryVelocity ?? 0;
+    if (velocity.abs() < _swipeVelocityThreshold) return;
+
+    if (velocity < 0 && _currentIndex < _pages.length - 1) {
+      _onTabSelected(_currentIndex + 1);
+      return;
+    }
+
+    if (velocity > 0 && _currentIndex > 0) {
+      _onTabSelected(_currentIndex - 1);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      body: IndexedStack(
-        index: _currentIndex,
-        children: _pages,
+      body: GestureDetector(
+        behavior: HitTestBehavior.translucent,
+        onHorizontalDragEnd: _handleHorizontalSwipe,
+        child: IndexedStack(
+          index: _currentIndex,
+          children: _pages,
+        ),
       ),
       bottomNavigationBar: NavigationBar(
         selectedIndex: _currentIndex,