feat: 管理后台数据看板

新增 /admin/dashboard 页面，提供充值订单统计与分析：
- 汇总统计卡片（今日/累计充值金额、订单数、成功率、平均充值）
- 每日充值趋势折线图（recharts，支持 7/30/90 天切换）
- 充值排行榜（Top 10 用户）
- 支付方式分布（水平条形图）
- 与 /admin 订单管理页面互相导航

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/admin/DashboardStats.tsx

@@ -0,0 +1,58 @@
+'use client';
+
+interface Summary {
+  today: { amount: number; orderCount: number; paidCount: number };
+  total: { amount: number; orderCount: number; paidCount: number };
+  successRate: number;
+  avgAmount: number;
+}
+
+interface DashboardStatsProps {
+  summary: Summary;
+  dark?: boolean;
+}
+
+export default function DashboardStats({ summary, dark }: DashboardStatsProps) {
+  const cards = [
+    { label: '今日充值', value: `¥${summary.today.amount.toLocaleString()}`, accent: true },
+    { label: '今日订单', value: `${summary.today.paidCount}/${summary.today.orderCount}` },
+    { label: '累计充值', value: `¥${summary.total.amount.toLocaleString()}`, accent: true },
+    { label: '累计订单', value: String(summary.total.paidCount) },
+    { label: '成功率', value: `${summary.successRate}%` },
+    { label: '平均充值', value: `¥${summary.avgAmount.toFixed(2)}` },
+  ];
+
+  return (
+    <div className="grid grid-cols-2 gap-3 sm:grid-cols-3 lg:grid-cols-6">
+      {cards.map((card) => (
+        <div
+          key={card.label}
+          className={[
+            'rounded-xl border p-4',
+            dark
+              ? 'border-slate-700 bg-slate-800/60'
+              : 'border-slate-200 bg-white shadow-sm',
+          ].join(' ')}
+        >
+          <p className={['text-xs font-medium', dark ? 'text-slate-400' : 'text-slate-500'].join(' ')}>
+            {card.label}
+          </p>
+          <p
+            className={[
+              'mt-1 text-xl font-semibold tracking-tight',
+              card.accent
+                ? dark
+                  ? 'text-indigo-400'
+                  : 'text-indigo-600'
+                : dark
+                  ? 'text-slate-100'
+                  : 'text-slate-900',
+            ].join(' ')}
+          >
+            {card.value}
+          </p>
+        </div>
+      ))}
+    </div>
+  );
+}