chore: 初始化脚手架 — sunny_mochi 企业级 Flutter 模板

Core 基础设施：Flavor 三包体系、Drift+SQLCipher 加密数据库
7 拦截器 Dio 网络栈、CrashReporter 崩溃日志、Sealed Failure 错误体系
5 色板主题系统、Auth 认证骨架（Clean Architecture）、Dev Panel、Mock Adapter

通过验证：flutter analyze（0 errors）、flutter test（全绿）、staging APK 74.8MB

lib/core/widgets/shell_scaffold.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+
+/// 底部导航 Shell — 2 Tab（Home / Mine）。
+/// Tab 标签文字在 Task 5 替换为 i18n 字符串。
+class ShellScaffold extends StatelessWidget {
+  const ShellScaffold({super.key, required this.shell});
+
+  final StatefulNavigationShell shell;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: shell,
+      bottomNavigationBar: NavigationBar(
+        selectedIndex: shell.currentIndex,
+        onDestinationSelected: (index) => shell.goBranch(
+          index,
+          initialLocation: index == shell.currentIndex,
+        ),
+        destinations: const [
+          NavigationDestination(
+            icon: Icon(Icons.home_outlined),
+            selectedIcon: Icon(Icons.home),
+            label: 'Home', // TODO: i18n → t.tab.home
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.person_outline),
+            selectedIcon: Icon(Icons.person),
+            label: 'Mine', // TODO: i18n → t.tab.mine
+          ),
+        ],
+      ),
+    );
+  }
+}