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)

backend/internal/handler/payment_handler.go

@@ -412,5 +412,10 @@ func requireAuth(c *gin.Context) (middleware2.AuthSubject, bool) {
 // isMobile detects mobile user agents.
 func isMobile(c *gin.Context) bool {
 	ua := strings.ToLower(c.GetHeader("User-Agent"))
-	return strings.Contains(ua, "mobile") || strings.Contains(ua, "android") || strings.Contains(ua, "iphone")
+	for _, kw := range []string{"mobile", "android", "iphone", "ipad", "ipod"} {
+		if strings.Contains(ua, kw) {
+			return true
+		}
+	}
+	return false
 }