🎫 feat: Enhance redemption code expiry handling & improve UI responsiveness

Backend
• Introduced `validateExpiredTime` helper in `controller/redemption.go`; reused in both Add & Update endpoints to enforce “expiry time must not be earlier than now”, eliminating duplicated checks
• Removed `RedemptionCodeStatusExpired` constant and all related references – expiry is now determined exclusively by the `expired_time` field for simpler, safer state management
• Simplified `DeleteInvalidRedemptions`: deletes codes that are `used` / `disabled` or `enabled` but already expired, without relying on extra status codes
• Controller no longer mutates `status` when listing or fetching redemption codes; clients derive expiry status from timestamp

Frontend
• Added reusable `isExpired` helper in `RedemptionsTable.js`; leveraged for:
  – status rendering (orange “Expired” tag)
  – action-menu enable/disable logic
  – row styling
• Removed duplicated inline expiry logic, improving readability and performance
• Adjusted toolbar layout: on small screens the “Clear invalid codes” button now wraps onto its own line, while “Add” & “Copy” remain grouped

Result
The codebase is now more maintainable, secure, and performant with no redundant constants, centralized validation, and cleaner UI behaviour across devices.

router/api-router.go

@@ -126,6 +126,7 @@ func SetApiRouter(router *gin.Engine) {
 			redemptionRoute.GET("/:id", controller.GetRedemption)
 			redemptionRoute.POST("/", controller.AddRedemption)
 			redemptionRoute.PUT("/", controller.UpdateRedemption)
+			redemptionRoute.DELETE("/invalid", controller.DeleteInvalidRedemption)
 			redemptionRoute.DELETE("/:id", controller.DeleteRedemption)
 		}
 		logRoute := apiRouter.Group("/log")