fix: localize registration error message

Replace hardcoded "Registration failed" with localized message
using new translation key 'user.auth.registrationFailed' with
more informative text.

Co-authored-by: kolaente <13721712+kolaente@users.noreply.github.com>

frontend/src/i18n/lang/en.json

@@ -68,7 +68,8 @@
       "alreadyHaveAnAccount": "Already have an account?",
       "remember": "Stay logged in",
       "registrationDisabled": "Registration is disabled.",
-      "passwordResetTokenMissing": "Password reset token is missing."
+      "passwordResetTokenMissing": "Password reset token is missing.",
+      "registrationFailed": "An error occurred during registration. Please check your input and try again."
     },
     "settings": {
       "title": "Settings",

frontend/src/views/user/Register.vue

@@ -266,10 +266,10 @@ async function submit() {
 				serverValidationErrors.value = fieldErrors
 			} else {
 				// Fallback to general error message if no field errors
-				errorMessage.value = e.message || 'Registration failed'
+				errorMessage.value = e.message || t('user.auth.registrationFailed')
 			}
 		} else {
-			errorMessage.value = 'Registration failed'
+			errorMessage.value = t('user.auth.registrationFailed')
 		}
 	}
 }