Throw proper error on duplicate emailing domain (#22790)

Adding an emailing domain that already exists blew up with a raw
QueryFailedError and the client just saw a generic "An error occurred".
The unique index on domain is global, so the workspace-scoped existence
check never caught rows owned by another workspace.

Now the check is unscoped and throws an EmailingDomainException mapped
to CONFLICT with a proper user-facing message, in both the
createEmailingDomain mutation and the email group channel flow. Also
dropped the hardcoded catch-all snackbar on the new channel page so
server messages actually reach the user.

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packages/twenty-server/src/engine/metadata-modules/message-channel/utils/message-channel-graphql-api-exception-handler.util.ts

@@ -1,6 +1,11 @@
 import { assertUnreachable } from 'twenty-shared/utils';
 
 import {
+  EmailingDomainException,
+  EmailingDomainExceptionCode,
+} from 'src/engine/core-modules/emailing-domain/exceptions/emailing-domain.exception';
+import {
+  ConflictError,
   ForbiddenError,
   InternalServerError,
   NotFoundError,
@@ -32,6 +37,16 @@ export const messageChannelGraphqlApiExceptionHandler = (error: Error) => {
     }
   }
 
+  if (error instanceof EmailingDomainException) {
+    switch (error.code) {
+      case EmailingDomainExceptionCode.EMAILING_DOMAIN_ALREADY_REGISTERED:
+        throw new ConflictError(error);
+      default: {
+        return assertUnreachable(error.code);
+      }
+    }
+  }
+
   if (error instanceof ConnectedAccountException) {
     switch (error.code) {
       case ConnectedAccountExceptionCode.CONNECTED_ACCOUNT_OWNERSHIP_VIOLATION: