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)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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