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/skeleton.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+
+/// 骨架屏占位矩形。
+///
+/// 用法（配合 shimmer 包）：
+/// ```dart
+/// Shimmer.fromColors(
+///   baseColor: Colors.grey[300]!,
+///   highlightColor: Colors.grey[100]!,
+///   child: SkeletonBox(width: double.infinity, height: 20),
+/// )
+/// ```
+class SkeletonBox extends StatelessWidget {
+  const SkeletonBox({
+    super.key,
+    this.width,
+    this.height = 14,
+    this.borderRadius = 6,
+  });
+
+  final double? width;
+  final double height;
+  final double borderRadius;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: width,
+      height: height,
+      decoration: BoxDecoration(
+        color: Theme.of(context).colorScheme.surfaceContainerHighest,
+        borderRadius: BorderRadius.circular(borderRadius),
+      ),
+    );
+  }
+}
+
+/// 骨架屏圆形（头像占位）。
+class SkeletonCircle extends StatelessWidget {
+  const SkeletonCircle({super.key, this.size = 40});
+
+  final double size;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: size,
+      height: size,
+      decoration: BoxDecoration(
+        color: Theme.of(context).colorScheme.surfaceContainerHighest,
+        shape: BoxShape.circle,
+      ),
+    );
+  }
+}