feat(payment): add H5/mobile payment support

Backend:
- Parse EasyPay `payurl2` field, prefer H5 link on mobile
- Add `device=mobile` to EasyPay submit.php (popup) mode
- Expand isMobile detection keywords (add ipad/ipod)

Frontend:
- Add `isMobileDevice()` utility (userAgentData + UA regex)
- Mobile + pay_url: direct redirect instead of QR/popup
- Popup blocked fallback: auto-redirect when window.open fails
- Stripe WeChat Pay: dynamic client param (mobile_web vs web)

frontend/src/utils/device.ts

@@ -0,0 +1,11 @@
+/**
+ * Detect whether the current device is mobile.
+ * Uses navigator.userAgentData (modern API) with UA regex fallback.
+ */
+export function isMobileDevice(): boolean {
+  const nav = navigator as unknown as Record<string, unknown>
+  if (nav.userAgentData && typeof (nav.userAgentData as Record<string, unknown>).mobile === 'boolean') {
+    return (nav.userAgentData as Record<string, unknown>).mobile as boolean
+  }
+  return /Android|iPhone|iPad|iPod|Mobile/i.test(navigator.userAgent)
+}