fix：UI对齐demo

lib/common/router/tab_shell.dart

@@ -101,7 +101,7 @@ class _TabShellState extends State<TabShell> {
       ),
       bottomNavigationBar: DecoratedBox(
         decoration: BoxDecoration(
-          color: cs.background,
+          color: Colors.white,
           border: Border(top: BorderSide(color: cs.border, width: 1)),
         ),
         child: SafeArea(
@@ -155,12 +155,26 @@ class _TabButton extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final cs = ShadTheme.of(context).colorScheme;
-    final color = selected ? cs.foreground : cs.mutedForeground;
+    final color = selected ? cs.primary : cs.mutedForeground;
     return InkWell(
       onTap: onTap,
       child: Column(
         mainAxisAlignment: MainAxisAlignment.center,
         children: [
+          if (selected)
+            Container(
+              width: 22,
+              height: 2.5,
+              margin: const EdgeInsets.only(bottom: 6),
+              decoration: BoxDecoration(
+                color: cs.primary,
+                borderRadius: const BorderRadius.vertical(
+                  bottom: Radius.circular(3),
+                ),
+              ),
+            )
+          else
+            const SizedBox(height: 8.5),
           Icon(icon, size: 22, color: color),
           const SizedBox(height: 4),
           Text(