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/lib/components/ui/Button.svelte

@@ -0,0 +1,60 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import LoaderCircle from '@lucide/svelte/icons/loader-circle';
+
+	interface Props {
+		variant?: 'primary' | 'secondary' | 'subtle' | 'danger';
+		size?: 'sm' | 'md' | 'lg';
+		type?: 'button' | 'submit';
+		href?: string;
+		reload?: boolean;
+		disabled?: boolean;
+		loading?: boolean;
+		onclick?: (event: MouseEvent) => void;
+		class?: string;
+		children: Snippet;
+	}
+
+	let {
+		variant = 'primary',
+		size = 'md',
+		type = 'button',
+		href,
+		reload = false,
+		disabled = false,
+		loading = false,
+		onclick,
+		class: klass = '',
+		children
+	}: Props = $props();
+
+	const sizes = {
+		sm: 'px-4 py-2 text-base',
+		md: 'px-5 py-2.5 text-base',
+		lg: 'px-8 py-3.5 text-base'
+	};
+
+	const variants = {
+		primary: 'bg-accent-soft font-semibold text-on-accent active:scale-[0.97]',
+		secondary: 'border border-line text-fg hover:bg-hover',
+		subtle: 'bg-surface-2 font-semibold text-fg hover:bg-surface-hover',
+		danger: 'bg-danger font-semibold text-white hover:brightness-110'
+	};
+
+	const classes = $derived(
+		`inline-flex items-center justify-center gap-2 rounded-control transition disabled:opacity-40 ${sizes[size]} ${variants[variant]} ${klass}`
+	);
+</script>
+
+{#if href}
+	<a {href} data-sveltekit-reload={reload ? '' : undefined} class={classes}>
+		{@render children()}
+	</a>
+{:else}
+	<button {type} disabled={disabled || loading} {onclick} class={classes}>
+		{#if loading}
+			<LoaderCircle class="h-4 w-4 animate-spin" />
+		{/if}
+		{@render children()}
+	</button>
+{/if}