fix: disable the sign up and sign in button when form is invalid (#7376)

## Summary

<!-- One sentence describing what changed and why. -->

Add a disabled state to the sign-up button in the cloud onboarding
views. The button should be disabled when the form is invalid to prevent
users from submitting incomplete or incorrectly formatted information.

## Changes

- **What**: <!-- Core functionality added/modified -->
- Add disabled state to SignUp button and SignIn button when SignUp or
SignIn form is invalid.
- **Breaking**: <!-- Any breaking changes (if none, remove this line)
-->
- **Dependencies**: <!-- New dependencies (if none, remove this line)
-->

## Review Focus

<!-- Critical design decisions or edge cases that need attention -->

Changes for this notion:
https://www.notion.so/comfy-org/Implement-Disable-sign-up-button-when-form-is-invalid-in-cloud-onboarding-2c56d73d365081edbf8bf06b1f5e52e5

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots (if applicable)
Sign In button

Before(button not disabled when email is invalid)
![Kapture 2025-12-11 at 22 30
59](https://github.com/user-attachments/assets/4278473b-350e-4fea-a299-199697c354b7)

After
![Kapture 2025-12-11 at 22 28
45](https://github.com/user-attachments/assets/b677a444-39ce-487c-a2ad-31369585e333)

<!-- Add screenshots or video recording to help explain your changes -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7376-fix-disable-the-sign-up-and-sign-in-button-when-form-is-invalid-2c66d73d36508139af44cd7cb1e1aeb9)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/components/dialog/content/signin/SignInForm.vue

@@ -66,6 +66,7 @@
       type="submit"
       :label="t('auth.login.loginButton')"
       class="mt-4 h-10 font-medium"
+      :disabled="!$form.valid"
     />
   </Form>
 </template>

src/components/dialog/content/signin/SignUpForm.vue

@@ -1,5 +1,6 @@
 <template>
   <Form
+    v-slot="$form"
     class="flex flex-col gap-6"
     :resolver="zodResolver(signUpSchema)"
     @submit="onSubmit"
@@ -34,6 +35,7 @@
       type="submit"
       :label="t('auth.signup.signUpButton')"
       class="mt-4 h-10 font-medium"
+      :disabled="!$form.valid"
     />
   </Form>
 </template>

src/platform/cloud/onboarding/components/CloudSignInForm.vue

@@ -69,6 +69,7 @@
       type="submit"
       :label="t('auth.login.loginButton')"
       class="mt-4 h-10 font-medium text-white"
+      :disabled="!$form.valid"
     />
   </Form>
 </template>