Loading states are shown during async setup operations

A submit button with no loading state invites double-clicks that create duplicate accounts, double-charge users, or race against each other to corrupt state. A post-signup redirect that shows a blank white screen for 1-2 seconds leaves the user wondering whether something broke and occasionally triggers a browser back button press that aborts the flow. The user-experience taxon flags both as foundational: any asynchronous operation without visible feedback degrades perceived reliability and invites user behavior that causes real bugs.

Disable submit buttons and swap their label while a submission is in flight, and add a loading.tsx file for any async route that can render blank on transition. Edit src/app/(auth)/signup/page.tsx to gate the button on an isPending flag, and create src/app/(auth)/loading.tsx with a spinner.

<Button type="submit" disabled={isPending} className="w-full h-11">
  {isPending ? 'Creating account...' : 'Create account'}
</Button>