fix(core): prevent double-submit on login flow grant page

[okxint]

Fixes #62327

The Vue rewrite of LoginFlowGrant dropped the disable-on-submit guard that existed in the old grant.js (added in #33164). Without it, Chrome's double-fire behavior or a fast second click sends two POST requests before the first response arrives, which can result in duplicate grant confirmations.

Fix: add an isSubmitting ref that blocks re-entry into onSubmit and disables the button while the request is in flight. A NcLoadingIcon is shown in the #icon slot during submission, following the same pattern used in PublicPageMenuExternalDialog.vue.

<!-- before -->
<NcButton type="submit" variant="primary">
    {{ t('core', 'Grant access') }}
</NcButton>

<!-- after -->
<NcButton :disabled="isSubmitting" type="submit" variant="primary">
    <template v-if="isSubmitting" #icon>
        <NcLoadingIcon />
    </template>
    {{ t('core', 'Grant access') }}
</NcButton>