feat: make FormRelay component feature-complete for Vue and Nuxt

[RobertBoes]

Summary

• Make publicKey optional on useFormRelay composable and <FormRelay> component — when omitted, schema fetch is skipped for manual form building
• Add initialSchema and botProtectionContainer as optional props on the Vue <FormRelay> component, matching features already available on the composable
• Replace the Nuxt <FormRelay> re-export with an async component wrapping the Nuxt useFormRelay composable, providing SSR schema prefetch, automatic publicKey from runtime config, and secret key support

Nuxt <FormRelay> usage

<FormRelay form-id="contact">
  <template #loading>
    <p>Loading form...</p>
  </template>
  <template #error="{ error }">
    <p>Failed to load: {{ error.detail }}</p>
  </template>
  <template #default="{ fields, values, errors, submit, canSubmit }">
    <!-- form fields + submit button -->
  </template>
</FormRelay>

Only formId is required — publicKey comes from runtime config automatically.

Test plan

• Composable skips schema fetch when publicKey is omitted
• Submit is a no-op without a schema
• Component accepts initialSchema prop and skips fetch
• Component renders immediately when publicKey is omitted
• All 57 existing + new tests pass
• Build succeeds for all packages
• Manual testing: link local packages into a real world Nuxt app