feat(user): support fixed UUID registration and enhance bulk import results

- Added support for fixed UUIDs during bulk registration (Search-first + ExternalID mapping)
- Implemented idempotency and visibility restoration for soft-deleted users
- Enhanced bulk upload UI to show 'New/Updated/Unchanged' status and modified fields
- Added logic to reclaim identifiers (login_id) from colliding records
- Added frontend E2E and backend unit tests for UUID integrity and conflict handling
- Fixed i18n, formatting, and mock tests to satisfy code-check
- Applied 'go fix' for 'omitzero' tags and general Go standards

backend/internal/response/error_response.go

@@ -3,16 +3,16 @@ package response
 import "github.com/gofiber/fiber/v2"
 
 type ErrorResponse struct {
-	Error   string      `json:"error"`
-	Code    string      `json:"code,omitempty"`
-	Details interface{} `json:"details,omitempty"`
+	Error   string `json:"error"`
+	Code    string `json:"code,omitempty"`
+	Details any    `json:"details,omitempty"`
 }
 
 func Error(c *fiber.Ctx, status int, code, message string) error {
 	return ErrorWithDetails(c, status, code, message, nil)
 }
 
-func ErrorWithDetails(c *fiber.Ctx, status int, code, message string, details interface{}) error {
+func ErrorWithDetails(c *fiber.Ctx, status int, code, message string, details any) error {
 	resp := ErrorResponse{
 		Error:   message,
 		Code:    code,