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/section_card.dart

@@ -0,0 +1,34 @@
+import 'package:flutter/material.dart';
+
+/// 带圆角边框的内容分组卡片。
+class SectionCard extends StatelessWidget {
+  const SectionCard({
+    super.key,
+    required this.child,
+    this.padding = const EdgeInsets.all(16),
+    this.margin = const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+  });
+
+  final Widget child;
+  final EdgeInsetsGeometry padding;
+  final EdgeInsetsGeometry margin;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      margin: margin,
+      decoration: BoxDecoration(
+        color: Theme.of(context).colorScheme.surface,
+        borderRadius: BorderRadius.circular(12),
+        boxShadow: [
+          BoxShadow(
+            color: Colors.black.withValues(alpha: 0.05),
+            blurRadius: 8,
+            offset: const Offset(0, 2),
+          ),
+        ],
+      ),
+      child: Padding(padding: padding, child: child),
+    );
+  }
+}