fix(login): pass callbackUrl so local auth redirects to app after sign-in

After a successful Credentials sign-in, Auth.js was redirecting to its
own built-in /auth/signin page because no callbackUrl was provided.
Add a callbackUrl hidden field (pointing to the app root) so Auth.js
redirects back to the application after local login completes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/login/+page.svelte

@@ -45,6 +45,7 @@
 			{#if data.localAuthEnabled}
 				<form method="POST" action={data.localSigninUrl} class="mt-6 space-y-4">
 					<input type="hidden" name="csrfToken" />
+					<input type="hidden" name="callbackUrl" value={data.callbackUrl} />
 					<div>
 						<label class="text-sm font-medium text-slate-700" for="identifier">
 							Username or email