feat(admin): rebuild the payment center overview with ECharts analytics

- Overview page: realtime today vs. same time yesterday, 6 KPI cards with
  period-over-period deltas, trend (paid / net / refund curves + order count
  line), category curves, payment method donut, top 10 paid content,
  conversion / close / refund rates, todos and shortcuts
- Date presets, custom range and category filter via
  GET aksrt/v1/admin/pay-overview (manage_options + wp_rest nonce)
- Metrics: paid amount by paid date including later-refunded orders, refunds
  by refund date, net = paid - refund; CNY only, points and recharge excluded,
  foreign-currency orders listed separately; integer cents internally
- Daily records cached per month in transients (today/yesterday live),
  invalidated on order status change and on purge
- ECharts 6.1.0 subset bundled to assets/js/runtime/libs/echarts-pay.min.js
  (npm run build:pay-charts), loaded only on the overview page
- Split payment-attempt methods out of OrderRepositoryCore into
  OrderRepositoryAttempts; remove dead get_stats(), cash_decimal(),
  Order::get_stats() and the old .aksrt-native-links styles

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/build-pay-charts.mjs

@@ -0,0 +1,29 @@
+/**
+ * 打包支付中心概览用的 ECharts 子集。
+ *
+ *   cd assets/admin && npm run build:pay-charts
+ *
+ * echarts 是 assets/admin 的 devDependency；产物入库，部署时无需安装 node 依赖。
+ */
+import { createRequire } from 'node:module';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const adminDir = path.join(repoRoot, 'assets/admin');
+const require = createRequire(path.join(adminDir, 'package.json'));
+const { build } = require('esbuild');
+
+const outfile = path.join(repoRoot, 'assets/js/runtime/libs/echarts-pay.min.js');
+
+await build({
+  entryPoints: [path.join(repoRoot, 'tools/echarts-pay-overview.entry.mjs')],
+  outfile,
+  bundle: true,
+  format: 'iife',
+  minify: true,
+  target: ['es2018'],
+  legalComments: 'inline',
+  nodePaths: [path.join(adminDir, 'node_modules')],
+  logLevel: 'info',
+});

tools/echarts-pay-overview.entry.mjs

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+/**
+ * 支付中心概览专用的 ECharts 按需入口（由 tools/build-pay-charts.mjs 打包）。
+ *
+ * 只注册概览页用到的图表与组件，打包为 IIFE 并挂到 window.echarts。
+ * 产物：assets/js/runtime/libs/echarts-pay.min.js（入库，后台只在 aksrt_pay 页入队）。
+ */
+import * as echarts from 'echarts/core';
+import { LineChart, BarChart, PieChart } from 'echarts/charts';
+import {
+  GridComponent,
+  TooltipComponent,
+  LegendComponent,
+  DataZoomComponent,
+  AxisPointerComponent,
+  AriaComponent,
+} from 'echarts/components';
+import { SVGRenderer } from 'echarts/renderers';
+
+echarts.use([
+  LineChart,
+  BarChart,
+  PieChart,
+  GridComponent,
+  TooltipComponent,
+  LegendComponent,
+  DataZoomComponent,
+  AxisPointerComponent,
+  AriaComponent,
+  SVGRenderer,
+]);
+
+window.echarts = echarts;