style: 导航栏去掉蓝色药丸背景，改为图标/文字高亮

- indicatorColor: transparent 去掉 Material 3 默认的选中背景
- 选中项：图标 + 文字改为 primary 紫色，字重 w600
- 未选中项：图标 + 文字灰色 (textSecondary)，字重 w400
- 与微信/支付宝/飞书的导航栏风格一致

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0_app/lib/core/theme/app_theme.dart

@@ -29,6 +29,30 @@ class AppTheme {
         borderSide: BorderSide.none,
       ),
     ),
+    navigationBarTheme: NavigationBarThemeData(
+      backgroundColor: AppColors.surface,
+      indicatorColor: Colors.transparent,
+      labelTextStyle: WidgetStateProperty.resolveWith((states) {
+        if (states.contains(WidgetState.selected)) {
+          return const TextStyle(
+            fontSize: 11,
+            fontWeight: FontWeight.w600,
+            color: AppColors.primary,
+          );
+        }
+        return const TextStyle(
+          fontSize: 11,
+          fontWeight: FontWeight.w400,
+          color: AppColors.textSecondary,
+        );
+      }),
+      iconTheme: WidgetStateProperty.resolveWith((states) {
+        if (states.contains(WidgetState.selected)) {
+          return const IconThemeData(color: AppColors.primary, size: 24);
+        }
+        return const IconThemeData(color: AppColors.textSecondary, size: 24);
+      }),
+    ),
   );
 
   static ThemeData get lightTheme => ThemeData(