make readonly and required mutually exclusive

per https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Attributes/readonly

"Note: The required attribute is not permitted on inputs with the readonly attribute specified."

dashboard/src/components/MailboxDialog.vue

@@ -167,7 +167,7 @@ defineExpose({
           <FormGroup>
             <label for="nameInput">{{ $t('email.addMailboxDialog.name') }}</label>
             <InputGroup>
-              <TextInput id="nameInput" style="flex-grow: 1;" v-model="name" :readonly="mailbox ? true : undefined" :required="!mailbox"/>
+              <TextInput id="nameInput" style="flex-grow: 1;" v-model="name" :readonly="!!mailbox" :required="!mailbox"/>
               <SingleSelect v-model="domain" :options="domainList" option-key="domain" option-label="label" :disabled="!!mailbox" :required="!mailbox"/>
             </InputGroup>
             <div class="warning-label" v-if="!domainHasIncomingEnabled">{{ $t('email.addMailboxDialog.incomingDisabledWarning') }}</div>