feat(app): 뒤로가기 두 번 눌러서 앱 종료

메인 화면에서 뒤로가기 1회 → '한 번 더 누르면 앱이 종료됩니다' 스낵바
2초 내 다시 누르면 앱 종료

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/android/app/src/main/AndroidManifest.xml

@@ -17,7 +17,8 @@
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
             android:hardwareAccelerated="true"
-            android:windowSoftInputMode="adjustResize">
+            android:windowSoftInputMode="adjustResize"
+            android:enableOnBackInvokedCallback="false">
             <!-- Specifies an Android theme to apply to this Activity as soon as
                  the Android process has started. This theme is visible to the user
                  while the Flutter UI initializes. After that, this theme continues

app/lib/views/main_shell.dart

@@ -2,23 +2,52 @@
 library;
 
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:go_router/go_router.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import '../core/constants.dart';
 
 /// 메인 앱 셸 (툴바 + 바텀 네비게이션 + 콘텐츠)
-class MainShell extends StatelessWidget {
+class MainShell extends StatefulWidget {
   final Widget child;
 
   const MainShell({super.key, required this.child});
 
+  @override
+  State<MainShell> createState() => _MainShellState();
+}
+
+class _MainShellState extends State<MainShell> {
+  DateTime? _lastBackPressed;
+
   @override
   Widget build(BuildContext context) {
+    final child = widget.child;
     final location = GoRouterState.of(context).uri.path;
     final isMembersPage = location == '/members';
     final isSchedulePage = location.startsWith('/schedule');
 
-    return Scaffold(
+    return PopScope(
+      canPop: false,
+      onPopInvokedWithResult: (bool didPop, dynamic result) {
+        if (didPop) return;
+
+        final now = DateTime.now();
+        if (_lastBackPressed != null &&
+            now.difference(_lastBackPressed!) < const Duration(seconds: 2)) {
+          SystemNavigator.pop();
+        } else {
+          _lastBackPressed = now;
+          ScaffoldMessenger.of(context).showSnackBar(
+            const SnackBar(
+              content: Text('한 번 더 누르면 앱이 종료됩니다'),
+              duration: Duration(seconds: 2),
+              behavior: SnackBarBehavior.floating,
+            ),
+          );
+        }
+      },
+      child: Scaffold(
       backgroundColor: AppColors.background,
       // 앱바 (툴바) - 일정 페이지는 자체 툴바 사용, 멤버 페이지는 그림자 제거
       appBar: isSchedulePage
@@ -60,6 +89,7 @@ class MainShell extends StatelessWidget {
       body: child,
       // 바텀 네비게이션
       bottomNavigationBar: const _BottomNavBar(),
+      ),
     );
   }