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/lib/components/Footer.svelte

@@ -5,7 +5,7 @@
 		<p>© {new Date().getFullYear()} Musify · Streaming de música.</p>
 		<div class="flex gap-6">
 			<a href="/explore" class="transition hover:text-neutral-300">Explorar</a>
-			<a href="/login" class="transition hover:text-neutral-300">Entrar</a>
+			<a href="/login" data-sveltekit-reload class="transition hover:text-neutral-300">Entrar</a>
 		</div>
 	</div>
 </footer>