Consolidate auth routes under /auth and rename /u to /user

Moves the login screen from /login to /auth, logout from /logout to
/auth/logout (alongside the existing /auth/login and /auth/callback
endpoints), and the public profile route from /u/[id] to /user/[id].

AUTH_REDIRECT_URI and AUTH_POST_LOGOUT_URI already point at
/auth/callback and / respectively, so no Zitadel config change is
needed for the auth move.

web-player/src/routes/auth/+page.svelte

@@ -0,0 +1,46 @@
+<script lang="ts">
+	import Button from '$lib/components/ui/primitives/Button.svelte';
+	import Logo from '$lib/components/ui/primitives/Logo.svelte';
+
+	let { data } = $props();
+
+	const returnParam = $derived(`returnTo=${encodeURIComponent(data.returnTo)}`);
+</script>
+
+<svelte:head>
+	<title>Iniciar sesión</title>
+	<meta name="description" content="Inicia sesión o crea una cuenta en Musify." />
+</svelte:head>
+
+<div class="relative grid min-h-screen place-items-center overflow-hidden bg-bg px-6 text-fg">
+	<div class="pointer-events-none absolute inset-0" style="background:var(--mf-ambient)"></div>
+
+	<div class="relative flex w-full max-w-4xl flex-col items-center gap-12 text-center">
+		<Logo size="lg" />
+
+		<div class="animate-enter flex flex-col items-center gap-5">
+			<h1
+				class="font-display text-[clamp(2.75rem,9vw,8rem)] leading-[1.05] font-semibold tracking-tight"
+			>
+				<span class="block">Tu música.</span>
+				<span class="block">Sin límites.</span>
+			</h1>
+			<p class="text-lg text-fg-2">Inicia sesión para continuar.</p>
+		</div>
+
+		<div class="flex w-full max-w-cta flex-col gap-3">
+			<Button href={`/auth/login?${returnParam}`} reload size="lg" class="w-full">
+				Iniciar sesión
+			</Button>
+			<Button
+				href={`/auth/login?mode=register&${returnParam}`}
+				reload
+				variant="secondary"
+				size="lg"
+				class="w-full"
+			>
+				Crear cuenta
+			</Button>
+		</div>
+	</div>
+</div>