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/count_down_button.dart

@@ -0,0 +1,60 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+/// 带倒计时的发送按钮（常用于短信验证码）。
+///
+/// 倒计时结束后自动恢复可点击状态。
+class CountDownButton extends StatefulWidget {
+  const CountDownButton({
+    super.key,
+    required this.onSend,
+    this.label = '发送验证码',
+    this.countingLabel = '重新发送',
+    this.seconds = 60,
+  });
+
+  final VoidCallback onSend;
+  final String label;
+  final String countingLabel;
+  final int seconds;
+
+  @override
+  State<CountDownButton> createState() => _CountDownButtonState();
+}
+
+class _CountDownButtonState extends State<CountDownButton> {
+  int _remaining = 0;
+  Timer? _timer;
+
+  bool get _isCounting => _remaining > 0;
+
+  void _start() {
+    widget.onSend();
+    setState(() => _remaining = widget.seconds);
+    _timer = Timer.periodic(const Duration(seconds: 1), (t) {
+      if (_remaining <= 1) {
+        t.cancel();
+        if (mounted) setState(() => _remaining = 0);
+      } else {
+        if (mounted) setState(() => _remaining--);
+      }
+    });
+  }
+
+  @override
+  void dispose() {
+    _timer?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return TextButton(
+      onPressed: _isCounting ? null : _start,
+      child: Text(
+        _isCounting ? '${widget.countingLabel}(${_remaining}s)' : widget.label,
+      ),
+    );
+  }
+}