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>

functions.php

@@ -227,6 +227,10 @@ if (class_exists('AKSRT\\Payment\\CommissionLedger')) {
 if (class_exists('AKSRT\\Payment\\InviteCode')) {
     \AKSRT\Payment\InviteCode::get_instance()->init();
 }
+// 支付中心统计的按天缓存：订单状态变化 / 删除时失效（需前后台都挂载，回调不走后台）
+if (class_exists('AKSRT\\Payment\\OrderAnalyticsDaily')) {
+    \AKSRT\Payment\OrderAnalyticsDaily::get_instance()->init();
+}
 if (class_exists('\AKSRT\Content\Pay')) {
     \AKSRT\Content\Pay::get_instance()->init();
 }