refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

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

@@ -0,0 +1,38 @@
+<script lang="ts">
+	import Button from '$lib/components/ui/Button.svelte';
+
+	let { data } = $props();
+
+	const returnParam = $derived(`returnTo=${encodeURIComponent(data.returnTo)}`);
+</script>
+
+<svelte:head>
+	<title>Iniciar sesión · Musify</title>
+	<meta name="description" content="Inicia sesión o crea una cuenta en Musify." />
+</svelte:head>
+
+<div class="grid min-h-screen place-items-center bg-bg px-6 text-fg">
+	<div class="flex w-full max-w-[440px] flex-col items-center gap-9 text-center">
+		<div class="text-3xl font-extrabold tracking-tight">Musify</div>
+
+		<div>
+			<h1 class="text-4xl font-extrabold tracking-tight">Tu música. Sin límites.</h1>
+			<p class="mt-3 text-base text-fg-2">Inicia sesión para continuar.</p>
+		</div>
+
+		<div class="flex w-full flex-col gap-5">
+			<Button href={`/auth/login?${returnParam}`} reload size="lg" class="w-full">
+				Iniciar Sesión
+			</Button>
+			<Button
+				href={`/auth/login?mode=register&${returnParam}`}
+				reload
+				variant="subtle"
+				size="lg"
+				class="w-full"
+			>
+				Crear Cuenta
+			</Button>
+		</div>
+	</div>
+</div>