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/DailyChart.tsx

@@ -0,0 +1,110 @@
+'use client';
+
+import { ResponsiveContainer, LineChart, Line, XAxis, YAxis, Tooltip, CartesianGrid } from 'recharts';
+
+interface DailyData {
+  date: string;
+  amount: number;
+  count: number;
+}
+
+interface DailyChartProps {
+  data: DailyData[];
+  dark?: boolean;
+}
+
+function formatDate(dateStr: string) {
+  const [, m, d] = dateStr.split('-');
+  return `${m}/${d}`;
+}
+
+function formatAmount(value: number) {
+  if (value >= 10000) return `¥${(value / 10000).toFixed(1)}w`;
+  if (value >= 1000) return `¥${(value / 1000).toFixed(1)}k`;
+  return `¥${value}`;
+}
+
+interface TooltipPayload {
+  value: number;
+  dataKey: string;
+}
+
+function CustomTooltip({
+  active,
+  payload,
+  label,
+  dark,
+}: {
+  active?: boolean;
+  payload?: TooltipPayload[];
+  label?: string;
+  dark?: boolean;
+}) {
+  if (!active || !payload?.length) return null;
+  return (
+    <div
+      className={[
+        'rounded-lg border px-3 py-2 text-sm shadow-lg',
+        dark ? 'border-slate-600 bg-slate-800 text-slate-200' : 'border-slate-200 bg-white text-slate-800',
+      ].join(' ')}
+    >
+      <p className={['mb-1 text-xs', dark ? 'text-slate-400' : 'text-slate-500'].join(' ')}>{label}</p>
+      {payload.map((p) => (
+        <p key={p.dataKey}>
+          {p.dataKey === 'amount' ? '金额' : '笔数'}: {p.dataKey === 'amount' ? `¥${p.value.toLocaleString()}` : p.value}
+        </p>
+      ))}
+    </div>
+  );
+}
+
+export default function DailyChart({ data, dark }: DailyChartProps) {
+  // Auto-calculate tick interval: show ~10-15 labels max
+  const tickInterval = data.length > 30 ? Math.ceil(data.length / 12) - 1 : 0;
+  if (data.length === 0) {
+    return (
+      <div className={['rounded-xl border p-6', dark ? 'border-slate-700 bg-slate-800/60' : 'border-slate-200 bg-white shadow-sm'].join(' ')}>
+        <h3 className={['mb-4 text-sm font-semibold', dark ? 'text-slate-200' : 'text-slate-800'].join(' ')}>每日充值趋势</h3>
+        <p className={['text-center text-sm py-16', dark ? 'text-slate-500' : 'text-gray-400'].join(' ')}>暂无数据</p>
+      </div>
+    );
+  }
+
+  const axisColor = dark ? '#64748b' : '#94a3b8';
+  const gridColor = dark ? '#334155' : '#e2e8f0';
+
+  return (
+    <div className={['rounded-xl border p-6', dark ? 'border-slate-700 bg-slate-800/60' : 'border-slate-200 bg-white shadow-sm'].join(' ')}>
+      <h3 className={['mb-4 text-sm font-semibold', dark ? 'text-slate-200' : 'text-slate-800'].join(' ')}>每日充值趋势</h3>
+      <ResponsiveContainer width="100%" height={320}>
+        <LineChart data={data} margin={{ top: 5, right: 20, bottom: 5, left: 10 }}>
+          <CartesianGrid stroke={gridColor} strokeDasharray="3 3" />
+          <XAxis
+            dataKey="date"
+            tickFormatter={formatDate}
+            tick={{ fill: axisColor, fontSize: 12 }}
+            axisLine={{ stroke: gridColor }}
+            tickLine={false}
+            interval={tickInterval}
+          />
+          <YAxis
+            tickFormatter={formatAmount}
+            tick={{ fill: axisColor, fontSize: 12 }}
+            axisLine={{ stroke: gridColor }}
+            tickLine={false}
+            width={60}
+          />
+          <Tooltip content={<CustomTooltip dark={dark} />} />
+          <Line
+            type="monotone"
+            dataKey="amount"
+            stroke={dark ? '#818cf8' : '#4f46e5'}
+            strokeWidth={2}
+            dot={{ r: 3, fill: dark ? '#818cf8' : '#4f46e5' }}
+            activeDot={{ r: 5 }}
+          />
+        </LineChart>
+      </ResponsiveContainer>
+    </div>
+  );
+}