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/Field.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+
+	interface Props {
+		label: string;
+		for?: string;
+		optional?: boolean;
+		class?: string;
+		hint?: Snippet;
+		children: Snippet;
+	}
+
+	let { label, for: forId, optional = false, class: klass = '', hint, children }: Props = $props();
+</script>
+
+<div class="flex min-w-0 flex-col {klass}">
+	<label for={forId} class="mb-2 block text-base font-medium text-fg-2">
+		{label}
+		{#if optional}<span class="text-muted">(Opcional)</span>{/if}
+	</label>
+	{@render children()}
+	{#if hint}
+		<div class="mt-2 flex items-center justify-between gap-3 text-sm text-muted">
+			{@render hint()}
+		</div>
+	{/if}
+</div>