fix(genex-mobile): 首页移除返回键并修复搜索栏溢出

SliverAppBar 添加 automaticallyImplyLeading: false，
防止 Tab 导航进入时显示返回键，并修复返回键占位导致的搜索栏 overflow 6px。

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

frontend/genex-mobile/lib/features/coupons/presentation/pages/home_page.dart

@@ -76,6 +76,7 @@ class _HomePageState extends State<HomePage> {
                 SliverAppBar(
                   floating: true,
                   pinned: false,
+                  automaticallyImplyLeading: false,
                   backgroundColor: AppColors.background,
                   elevation: 0,
                   toolbarHeight: 60,