fix(frontend): full-page nav for auth links; land on home after login

- Add data-sveltekit-reload to the /login links so the browser performs a full
  navigation (the client router intercepted them and aborted on the external
  Zitadel redirect)
- Default post-login returnTo to / (home) since /explore is not built yet
- 'Empezar gratis' now starts the register flow (/login?mode=register)

frontend/src/routes/+page.svelte

@@ -45,7 +45,8 @@
 
 		<div class="mt-10 flex flex-wrap items-center justify-center gap-4">
 			<a
-				href="/login"
+				href="/login?mode=register"
+				data-sveltekit-reload
 				class="rounded-full bg-emerald-500 px-6 py-3 text-sm font-semibold text-neutral-950 transition hover:bg-emerald-400"
 			>
 				Empezar gratis

frontend/src/routes/auth/callback/+server.ts

@@ -16,7 +16,7 @@ export const GET: RequestHandler = async ({ url, cookies }) => {
 	const codeVerifier = cookies.get(VERIFIER_COOKIE);
 	const state = cookies.get(STATE_COOKIE);
 	const nonce = cookies.get(NONCE_COOKIE);
-	const returnTo = cookies.get(RETURN_COOKIE) ?? '/explore';
+	const returnTo = cookies.get(RETURN_COOKIE) ?? '/';
 
 	if (!codeVerifier || !state) {
 		error(400, 'Estado de autenticación inválido. Vuelve a iniciar sesión.');

frontend/src/routes/login/+server.ts

@@ -20,7 +20,7 @@ export const GET: RequestHandler = async ({ url, cookies }) => {
 	const nonce = client.randomNonce();
 
 	const register = url.searchParams.get('mode') === 'register';
-	const returnTo = url.searchParams.get('returnTo') ?? '/explore';
+	const returnTo = url.searchParams.get('returnTo') ?? '/';
 
 	const parameters: Record<string, string> = {
 		redirect_uri: authConfig.redirectUri,