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/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>

frontend/src/lib/components/Navbar.svelte

@@ -48,11 +48,16 @@
 					</button>
 				</form>
 			{:else}
-				<a href="/login" class="text-sm font-medium text-neutral-300 transition hover:text-white">
+				<a
+					href="/login"
+					data-sveltekit-reload
+					class="text-sm font-medium text-neutral-300 transition hover:text-white"
+				>
 					Iniciar sesión
 				</a>
 				<a
 					href="/login?mode=register"
+					data-sveltekit-reload
 					class="rounded-full bg-emerald-500 px-4 py-2 text-sm font-semibold text-neutral-950 transition hover:bg-emerald-400"
 				>
 					Registrarse